Selecting the Trigger Type for a pad in the TD-50 optimizes the sensitivity settings for the trigger input. The "PAD 1-3" Trigger Types generally work best when connecting non-Roland brand electronic pads. Experiment with each of the three types, and see which one produces the best triggering results from the pad that you have connected.
Each of the "PAD 1-3" Trigger Types are not specific to any particular brand/model.