a) Total Commission is the amount of commissions paid to the agency by the manufacturer.
b) Agency Split is the amount credited within the agency to itself, or the "House", when the sales rep is paid some portion (the split) of the commissions paid to the agency by the manufacturer.
c) Sales Rep Split is the amount that the sales rep is paid, his portion of the commissions paid by the manufacturer.
These split cases are only used when you pay the sales rep a split of the commissions. For example, a salaried sales rep may not be paid any split of the commissions, and so his split rate would be set to 0 (0%).
There are 3 places that the split rate can be set: d) Sales Rep Management, e) Customer Listing, and f) Invoice Entry. Those three places are listed in order that they take effect, for example a setting in Invoice Entry overrides any setting in Customer Listing or Sales Rep Managmenet; a setting in Customer Listing overrides a Sales Rep Management entry; and a Sales Rep Management entry is the default for all cases where it is not overidden in one of these other places.