That's the witness mark. The stock tailcap also has one, and the pair of them are used to switch between constant on, momentary or locked-out at a glance.
That is the stock tailcap, but I think I understand now...
I guess I should have been more specific - this is a 6P Defender. So I'm guessing the standard 6PL or the incan 6P uses a twisty - this is just a forward clicky...