Airgun Flatline filling

[Wang74]

I am new to using HPA and used co2 before and I just got a new Flatline air system. I was just wondering how you fill the system. is it the thing at the end of the reg with the nut on it? or the thing labeled Hp/tank? if it's not the thing on the end of it with the nut, what is it for?
Sorry for the dumb ?'s but i'm just not very technically advanced.
Thanks
CW

[PyRo]

You know what a quick disconnect looks like? Thier is what looks like a male end of one around the reg on your tank. A female end connected to a tank, compressor, or booster is attached to this and used to fill it.

[Hexis]

Here, a pic is worth many words:

This is for something different, so it's a Flatline 4500 psi system. But you will get the idea.