GetSpellUsageNum

This condition function was introduced in Skyrim; it runs on an Actor and accepts a Spell as its argument. Its purpose and return value are not known at this time.

Disassembly indicates that it retrieves one of the magic effects from the input spell (though we don't know which effect yet). The function then loops over all active effects on the subject reference, and sums the magnitudes of any active effect that uses the same base effect that was retrieved from the spell. The sum of those magnitudes is the function's return value.