Create - Not allowed processing constraint - will it fire before creating the order line or will it

we have a requirement to not allow user to create an order line if customer po line number(at order lines) is not entered.

For this i have setup a processing constraint as Create - Not allowed with the validation template being an API call.

Now went to the sales order form, created the order header, go to lines tab, click create new record, this processing constraint is immediately firing.

I wanted to understand, if Create - Not allowed constraint will fire immediately when you try to create the record or will it fire after you enter all the order line details and then save it?