soft pay would be like deadheads and Extra pay in duty rigs.
For example, if you fly a one day trip that paid 3 hours, but min rig is 5, then you have 2 hours of soft pay. OR if you fly a dead head and get paid 5 hours, you have 5 hours of soft time.

It is basically time you’re getting paid for, but doesn’t count against your FAA limits.