The short answer
If the pool is empty but its period is still active, CALC does not automatically switch to remaining personal credits. Paid actions may be blocked. Personal credit purchases do not automatically refill the pool. An expired pool is no longer selected. Without another active pool, personal billing applies again. At the regular monthly renewal, the pool receives its new budget and usage counting restarts. Unused monthly credits do not accumulate. If renewal is missing, have the billing period checked.