You can use it however you want really. The intention is that "source" refers to the source of magic (wizard, cleric, specific patrons, etc.) - basically the same as 5E. School is not much of a thing in DCC, but if you have the level 2 wizard spell arcane affinity, there is potential to introduce schools of magic (conjuration, divination, etc.), so that would be the school of course. In most cases, that field is blank.

