When you as a race director move someone from one event to another, you now have the flexibility to give them credit for that transfer or not. For example if I transfer someone from the Half to the Full Marathon and the full marathon costs $20 extra. Some race directors want to make that $20 available to the runner for future refunds or transfers.
Note the self-serve functions still work the same and we calculate based on current even costs and amount paid. So if a runners paid the $20 event difference, then the $20 would be available for future refunds or transfers. The issue is that sometimes when a race director does this, they might not collect money from the runner so there is no need to have an amount that is potentially creditable to the runner.