The Expansion case is a range (or difference) case. Assume you have two operating conditions:

1) W + P1 + T1
2) W + P2 + T2

The expansion case (L1-L2) is the range between these two conditions. Some piping codes do not consider a change in pressure as a component of this range case, while other piping codes do. The configuration directive you asked about can be used to alter the default behavior.

You can see this be setting up a small job with P1= 125 and P2=75. Setup three load cases:

1) P1 SUS
2) P2 SUS
3) L1-L2 EXP

Set this configuration directive to "Yes", then analyze the system and check the stress reports for all three case.

Now change the configuration directive to "No" and reanalyze the system. Note the change in the stress report for the EXP case.

Changing the configuration directive to "Default" will match one of the earlier two runs. As to which one matches depends on the piping code you analyze to. For some codes Default=Yes and for other codes Default=No.