A credit is deducted each time an assessment email invitation is sent to a candidate. If a candidate drops out of an assessment before finishing the first test, the credit can be reclaimed.
For example, say you create an assessment with 2 tests. You then invite 10 candidates to the assessment. 10 credits are deducted immediately. 2 candidates drop out midway through Test 1, 3 candidates finish Test 1 and drop out during Test 2. 5 candidates finish both tests. You can then reclaim 2 credits and will only be charged 8 credits.