From FIXwiki

FIX Message Logon (MsgType=A)

MsgType	FIX Specification	First introduced	Deprecated
MsgType=A	Session messages - Session	FIX.2.7

The logon message authenticates a user establishing a connection to a remote system. The logon message must be the first message sent by the application requesting to initiate a FIX session.

The HeartBtInt (108) field is used to declare the timeout interval for generating heartbeats (same value used by both sides). The HeartBtInt value should be agreed upon by the two firms and specified by the Logon initiator and echoed back by the Logon acceptor.

Upon receipt of a Logon message, the session acceptor will authenticate the party requesting connection and issue a Logon message as acknowledgment that the connection request has been accepted. The acknowledgment Logon can also be used by the initiator to validate that the connection was established with the correct party.

The session acceptor must be prepared to immediately begin processing messages after receipt of the Logon. The session initiator can choose to begin transmission of FIX messages before receipt of the confirmation Logon, however it is recommended that normal message delivery wait until after the return Logon is received to accommodate encryption key negotiation.

The confirmation Logon can be used for encryption key negotiation. If a session key is deemed to be weak, a stronger session key can be suggested by returning a Logon message with a new key. This is only valid for encryption protocols that allow for key negotiation. (See the FIX Web Site"s Application notes for more information on a method for encryption and key passing.)

The Logon message can be used to specify the MaxMessageSize supported (i.e. can be used to control fragmentation rules for very large messages which support fragmentation). It can also be used to specify the MsgTypes supported for both sending and receiving.

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.