ARCA rail?

It’s a rail mounting system. It came from the photography industry. The standard is from Really Right Stuff (RRS) with a 1.5” system, but now everyone is making them.

It starts with a rail attached to your gun that can manufactured by different people. Area419 is probably the common one. From there, you get arca plates and accessories to mount on a tripod arca head.

I’m sure a search of YouTube or Google would give you a better understanding.
 
Here is an example of a RRS clamp attached to a bipod with a rail from MDT made to the RRS standard.
Screenshot_20200528-174433_Gallery.jpg
 
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Also, it makes it super quick and easy to slide attachments around the rail to various positions. Mainly used in PRS/NRL style comps. Plenty of info on YouTube about it...
 
It came with the screws, and I just put one in every hole. (That comment might fit in the MPT better, but you know what I mean). I just figured I’d use them all with a dab of blue loc tite, and I’d never have to worry about it, ever......