MT - It's SL's excellent (IMO) feature which allows non-volatile, EASY user configuration of lights to select one of several available basic, common sense, popular "UI" config settings, without getting into to the 'insanity' (IMO) of overly-complex / convoluted configurability options and 'fluff' that many manufacturers get into - and without having to carry a flow chart around to do it like some. The options typically include several basic settings for 'High output only', 2 outputs (like a low and a high), an option that includes strobe, etc., and choices can vary between specific models. I believe all include a 'high-only' and 2-output (selected by 'tap / double tap' at turn-on') option at minimum though, which covers my needs. I like a high and low option for general miscellaneous uses, but of course 'high only' in TAC / TAC backup role. One light handles both with Ten-Tap.
I never watched this video until now, but it appears to provide a decent explanation (and demo). Run this search and select 'Videos':
ASP provides a very similar feature in many of the lights in their product line. ASP has a "High-Low" setting that works for me any time, for example.
My Elzetta Alpha Gen 3 also quite simple, but provides access to a 'Hi' and a 'Low' with a rotary tail cap setting.