What is validation_level_code in table iby_validation_sets_b?

I am writing a custom validation and have created an entry in iby_validation_sets_b as indicated in the Payables Implementation manual.  In that manual, it shows an example where the validation_level_code is set to PAYMENT.  When I look at existing values for that field in the table, I see there are other values already used by other validations such as DOCUMENT.  So where is an explanation of the values that can be inserted in this field and what the implact of a value is?  I modified my validation from PAYMENT to DOCUMENT to try to get the validation to fire earlier