PT - Payload type.

A 3 bit field in the ATM cell header that indicates whether a cell
contains user information or Connection Associated Layer Management
information in its payload and also whether a cell experienced
congestion. Its currently defined values are:


000 User data cell, congestion not experienced, SDU-type = 0
001 User data cell, congestion not experienced, SDU-type = 1
010 User data cell, congestion experienced, SDU-type = 0
011 User data cell, congestion experienced, SDU-type = 1

100 Segment OAM F5 flow related cell
110 Reserved for future traffic control and resource management

111 Reserved for future functions

See also - ATM UNI Cell Structure