While you can change the instrument that's assigned to the head zone on each pad in each Kit as desired, the rim on the pad is assigned automatically based on the head instrument that has been chosen.
For example, if tom instrument " 12" C Maple" is assigned to the head of a tom pad, then the 12" C Maple rim shot instrument will automatically be assigned to the rim. It's not possible to assign the rims on the pads independently in the TD-25.