When VTAM® owns the PLU for the session, VTAM has the option to request compression. VTAM does not request compression if any of the following conditions occur:

• The CMPVTAM start option is set to 0, which indicates no compression.
• The CMPAPPLI and CMPAPPLO operands on the APPL definition statement are set to 0, which indicates no compression.
• The SLU owner prohibits compression through bits in the control vector X'66'. Whether the control vector indicates that compression is prohibited or required is determined by the setting of the COMPRES operand of the MODEENT macroinstruction.

If none of the previous conditions occur, the PLU requests compression levels. The PLU-to-SLU compression level is the lower of the values set on the CMPVTAM start option or the CMPAPPLO operand on the APPL definition statement. The SLU-to-PLU compression level is the lower of the values set on the CMPVTAM start option or the CMPAPPLI operand on the APPL definition statement.