If you choose a class length equal to the length of the smallest message sent in a transport class, XCF has the maximum flexibility to dynamically make adjustments to larger buffer sizes.

If you choose a class length equal to the length of the largest message sent in the transport class, XCF does not adjust buffer sizes because XCF never provides a message buffer smaller than defined message buffer size.

The smallest message buffer that XCF uses is large enough to accommodate the defined class length. If the class length is equal to the length of the largest message sent in the class, then the smallest buffer XCF uses can always accommodate the largest message. The advantage is that no message ever needs a larger message buffer and therefore never incurs the extra overhead necessary for XCF to prepare larger buffers.

It is possible that the algorithm used to adjust buffer size does not respond to the particular message traffic pattern at your installation. In this case, you may need to choose a larger class length. The disadvantage of choosing a larger class length is that storage might be wasted by messages that don't need such a large buffer.

When you choose a class length, keep in mind that the class length can impact the amount of message buffer space used for incoming messages (MAXMSG capacity). A large class length implies both larger message buffers and fewer message buffers for a given MAXMSG. Thus, increasing the class length could lead to less capacity to receive on target systems. As a rule of thumb, consider allowing at least 30 message buffers for the PATHIN buffer pool at all times.