• CLASS(BIG) - specifies BIG as the name of the transport class.
• CLASSLEN(40000) - specifies that messages to be handled by this class are up to 40,000+ bytes long. This specification causes MVS™ to optimize its processing for messages of up to (slightly more than) 40,000 bytes.

MVS defines the DEFAULT transport class automatically; it is not shown. MVS uses a class length of 956 for class DEFAULT; this length is sufficient for the small messages. Because no group is specifically assigned to any transport class, UNDESIG refers to all groups.

The CLASSDEF statement (for class BIG and class DEFAULT) segregate messages into two categories. Messages that are less than or equal to 956 bytes are sent using resources assigned to the transport class, DEFAULT. Messages between 956 bytes and 40,000+ bytes are sent using resources assigned to the BIG transport class. Note that messages longer than 40,000+ bytes are subject to performance degradation.