Linear Commission Schedule Based On Class per period

Let me start by saying we do not have quotas established in our system (we don't use them).

I have assigned each of our inventory items to one of four classes.

I then set up four linear Commission Schedules (based on sales per period (monthly) categorized by Class with the eligible amount=collections and rate = %). In each schedule I have 5 columns. If someone sells $0, the payout is 0%, $100 the payout is 3%, $251, 5%, 351, 7%, and 352, 7% (the amounts and % vary in each of the 4 schedules).

I'm a little confused by the help for the "Categorization Applies to" field. There are 2 choices, payout only or "both attainment and payout". Since we don't use quotas I'm not sure which to choose. I have been using "both attainment and payout" and have been seeing issues such as negative commissions for an employees check for no apparent reason (ie they did not go under one of the amount thresholds via a credit memo, etc.). I'm kind of hoping that in lieu of quotas it is using my amounts in the 5 columns referenced above....