I have the Millenium MPS600 drum module. The manual is a bit poor in terms of detailed information about the trigger settings. From experimentation and looking at manuals ( ie it's own as well as manuals for similar Roland and Yamaha gear ) I have figured out what most of them do, but to me 'Mask Time' ( Mask-T) and Reject time (Rjct-T) ... at least I assume it's Reject Time ... seem to be the same or very similar ... or at least I don't know what Rjct-T is.
all the manual says is ..
Rjct-T This parameter affects the suppression time, i.e. the response time of the trigger after playing it.