Baseball rules can be found here: http://mlb.mlb.com/mlb/official_info/official_rules
This document describes the baseball match state contract.

Match Summary Message

The Match Summary message complements the Match Details message by summarising the important match information for rapid, easy access.

{
"BatterPlayerId":389877,
"CustomerId":6105,
"FirstBasePlayerId":456210,
"FirstBaseOccupied":false,
"FixtureId":4462599,
"IsReliable":true,
"MessageTimestampUtc":"2017-03-08T13:29:14.52Z",
"Phase":InProgress,
"MatchDuration": null,
"WinningPitcher": null,
"LosingPitcher": null,
"SavingPitcher": null,
"BallInPlay":Unknown,
"PitchCount":{},
"PitcherPlayerId":5884211,
"SecondBasePlayerId":456211,
"SecondBaseOccupied":false,
"ThirdBasePlayerId": 456212,
"ThirdBaseOccupied":false,
"TotalErrors": {},
"TotalHits": {},
"TotalRuns": {},
"Innings": {}
}

Element	Type	Description
BatterPlayerId	String	An immutable identifier for the batter
CustomerId	Integer	An immutable identifier for the customer
FirstBasePlayerId	String	An immutable identifier for the first base
FirstBaseOccupied	Boolean	True if the base has been occupied
FixtureId	String	An immutable identifier for the event
IsReliable	Boolean	True if all summarised information is reliable. False if at least one of the summarised collections is unreliable.
MessageTimestampUtc	Datetime	The time that the message is sent, UTC
Phase	MatchPhase	The phase of play during which the action happened, possible values are: PreGame InProgress PostGame Suspended Cancelled Postponed Abandoned
BallInPlay	Enum	Indicates whether the ball is in play or not. Possible values are: Yes No Unknown (Default)
PitchCount	PitchCount	A collection of PitchCount objects that summarises the pitch count
PitcherPlayerId	String	An immutable identifier for the pitcher
SecondBasePlayerId	String	An immutable identifier for the second base
SecondBaseOccupied	Boolean	True if the base has been occupied
ThirdBasePlayerId	String	An immutable identifier for the third base
ThirdBaseOccupied	Boolean	True if the base has been occupied
TotalErrors	Score	A collection of TotalErrors objects that summarises the total errors
TotalHits	Score	A collection of TotalHits objects that summarises the total hits
TotalRuns	Score	A collection of TotalRuns objects that summarises the total runs
Innings	ArrayOfInningDetails	A collection of Innings objects that summarises the inning details
MatchDuration		Not currently used (null)
WinningPitcher		Not currently used (null)
LosingPitcher		Not currently used (null)
SavingPitcher		Not currently used (null)

PitchCount

A collection of objects describing Pitch.

{
"Balls":1,
"Strikes":2,
"TotalPitches":3
}

Element	Type	Description
Balls	Integer	Describes total balls by the current Pitcher
Strikes	Integer	Describes Strikes by the current Pitcher
TotalPitches	Integer	Describes total pitches by the current Pitcher

Score

A collection of objects describing Score.

{
"Home":0,
"Away":0
}

Element	Type	Description
Home	Integer	Integer count of Home total errors
Away	Integer	Integer count of Away total errors

ArrayOfInningDetails

A collection of objects describing the Inning

{
"Errors":{},
"Hits":{},
"InningNumber":1,
"LeftOnBase":{},
"Outs":{},
"Runs":{},
"Status": Finished
}

Element	Type	Description
Errors	Score	A collection of Errors objects that summarises the total errors
Hits	Score	A collection of Hits objects that summarises the total hits
InningNumber	Integer	Describes the inning played
LeftOnBase	Score	A collection of LeftOnBase objects that summarises the players on base
Outs	Score	A collection of Outs objects that summarises the total outs
Runs	Score	A collection of Runs objects that summarises the total runs
Status	InningStatus	Describes the Inning status, possible values are: Top Middle Bottom Finished
Top		PlateAppearances StartedAtUtc FinishedAtUtc Substitutions NumberOfOuts Runs PitcherStatistics
Bottom		PlateAppearances StartedAtUtc FinishedAtUtc Substitutions NumberOfOuts Runs PitcherStatistics

Match Details Message

The Match Details message contains the details of the game so far, including points scored, errors committed and runs.

{
"BatterPlayerId":512331,
"CustomerId":6105,
"FirstBasePlayerId":456210,
"FirstBaseOccupied":false,
"FixtureId":4462599,
"IsReliable":true,
"MessageTimestampUtc":"2017-03-08T13:29:14.52Z",
"Phase":PreGame,"MatchDuration": null,
"WinningPitcher": null,
"LosingPitcher": null,
"SavingPitcher": null,
"BallInPlay":Unknown,
"PitchCount":{},
"PitcherPlayerId":5884211,
"SecondBasePlayerId":456211,
"SecondBaseOccupied":false,
"ThirdBasePlayerId": 456212,
"ThirdBaseOccupied":false,
"TotalErrors": {},
"TotalHits": {},
"TotalRuns": {},
"AwayBattingLineup": {},
"AwayPlayers": {},
"AwayStartingLineup": {},
"HomeBattingLineup": {},
"HomePlayers": {},
"HomeStartingLineup": {},
"Innings": {}
}

Element	Type	Description
BatterPlayerId	String	An immutable identifier for the batter
CustomerId	Integer	An immutable identifier for the customer
FirstBasePlayerId	String	An immutable identifier for the first base
FirstBaseOccupied	Boolean	True if the base has been occupied
FixtureId	String	An immutable identifier for the event
IsReliable	Boolean	True if all summarised information is reliable. False if at least one of the summarised collections is unreliable.
MessageTimestampUtc	Datetime	The time that the message is sent, UTC
Phase	MatchPhase	The phase of play during which the action happened
BallInPlay	Enum	Indicates whether the ball is in play or not. Possible values are: Yes No Unknown (Default)
PitchCount	PitchCount	A collection of PitchCount objects that summarises the pitch count
PitcherPlayerId	String	An immutable identifier for the pitcher
SecondBasePlayerId	String	An immutable identifier for the second base
SecondBaseOccupied	Boolean	True if the base has been occupied
ThirdBasePlayerId	String	An immutable identifier for the third base
ThirdBaseOccupied	Boolean	True if the base has been occupied
TotalErrors	TotalErrors	A collection of TotalErrors objects that summarises the total errors
TotalHits	TotalHits	A collection of TotalHits objects that summarises the total hits
TotalRuns	TotalRuns	A collection of TotalRuns objects that summarises the total runs
AwayBattingLineup	BattingLineup	A collection of BattingLineup objects that summarises batting line up
AwayPlayers	ArrayOfPlayer	An Array Of Player objects that summarises players in team
AwayStartingLineup	ArrayOfLineupEntry	An Array Of Player objects that summarises staring line up for a team
HomeBattingLineup	BattingLineup	A collection of BattingLineup objects that summarises batting line up
HomePlayers	ArrayOfPlayer	An Array Of Player objects that summarises players in team
HomeStartingLineup	ArrayOfLineupEntry	An Array Of Player objects that summarises staring line up for a team
Innings	ArrayOfInningDetails	A collection of Innings objects that summarises the inning details
MatchDuration		Not currently used (null)
WinningPitcher		Not currently used (null)
LosingPitcher		Not currently used (null)
SavingPitcher		Not currently used (null)

BattingLineup

A collection of objects describing Batting line up

{
"CurrentBatterId":621637,
"LineupEntry":6105
}

Element	Type	Description
CurrentBatterId	String	An immutable identifier for the current batter
LineupEntry	ArrayOfLineupEntry

ArrayOfLineupEntry

A collection of objects describing line up

{
"BattingSlot":1,
"PlayerId":551022,
"Position":ShortStop
}

Element	Type	Description
BattingSlot	Integer	Player order for the batting slot
PlayerId	String	An immutable identifier for the pitcher
Position	PlayerPosition	Player position, possible values: Unknown Pitcher Catcher FirstBase SecondBase ThirdBase ShortStop LeftField CenterField RightField PinchHitter DesignatedHitter PinchRunner

ArrayOfPlayer

A collection of objects describing players

{
"BattingHand":Left,
"PlayerId":551022,
"Name":Stephen Drew,
"SourceId":null,
"ThrowingHand":Right 
}

Element	Type	Description
BattingHand	Hand	Describes players batting hand, possible values are: Unknown Right Left
PlayerId	String	An immutable identifier for the pitcher
Name	String	Player name
SourceId	String	An immutable identifier for the pitcher
ThrowingHand	Hand	Describes players throwing hand, possible values are: Unknown Right Left