As part of the session-establishment process, the PLU and the SLU must agree on the communication rules to be followed during the session. These communication rules, which are governed by session parameters, enable each end of the session to know what the other end of the session does and does not do in different communication situations.

Session parameters are indicators and numeric values that specify such things as "the PLU requires responses to requests" or "the SLU does not send end brackets if brackets are used." See What is communicated: Requests and responses, for a description of session parameters, and Specifying a session parameter, for an example of session parameter fields.