From FIXwiki

FIX Message TradingSessionList (MsgType=BJ)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=BJ	PreTrade messages - MarketStructureReferenceData	FIX.4.4

The Trading Session List message is sent as a response to a Trading Session List Request. The Trading Session List should contain the characteristics of the trading session and the current state of the trading session.

The message could be relayed every trading day, or at least when trading sessions are changed. The user of the message has the ability to relay either Trading Sessions only or, if applicable, Trading SubSessions. Depending on characteristics of the market, the various Time fields may apply.

The Trading Session List should return the TradSesReqID(tag 335) value from the Trading Session List Request originally sent by a counterparty.

Message Contents By FIX Version

Notes

Please post comments, clarifications, examples here. Click on the edit button on the right and away you go! 

Don't forget to refer to the FPL Discussion Groups to back up anything you write.
You will often find the authoritative statements on matters of FIX specification interpretation there.
Links from FIXwiki pages to relevant discussions on the FPL site are very valuable.

For example if you were writing a clarification on how to specify the "quantity" of an order in FIX, you might add the
following relevant link, http://www.fixprotocol.org/discuss/read/f5014573, to back up your points.