From FIXwiki

FIX Message TradingSessionListUpdateReport (MsgType=BS)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=BS	PreTrade messages - MarketStructureReferenceData	FIX.5.0

The Trading Session List Update Report is used by marketplaces to provide intra-day updates of trading sessions when there are changes to one or more trading sessions.

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Notes

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