Assign To Exclusive Group: 2

Assigns the currently selected pad to exclusive group 2. There are 8 exclusive groups.

The description of exclusive groups using the hi-hat as an example explains the concept succinctly and clearly.

Use Logic Pro Drum Machine Designer pad controls — Apple Support

Exclusive Group: Choose a group for the pad. You can assign multiple pads to the same group. As soon as one drum sound in the group is triggered, all other sounds in that same group are stopped. For example, you could group three pads with open, semi-closed, and closed hi-hat sounds. Only one of these sounds can be played at a time, mirroring the behavior of real hi-hats.