Correct: Tamiya X- and XF-series paints are acrylics, and X is gloss (or semi-gloss) while XF is flat/matt.
Though that's not what the question is about
Let's start with the simple ones: X-22 Clear (not XF-22, that's a medium grey) is simply gloss varnish. Use it to protect the paintwork underneath, or to make that glossy if it should be, but isn't. XF-86 is Flat Clear, not Flat Base — same idea as X-22, but use it to make the underlying paintwork matt (flat, if you prefer) when it's glossy.
The important one here is that X-21 Flat Base is not a paint and not a varnish! You would not be the first to think it's matt varnish, try to use it as such, and end up with a model with a powdery white sheen all over it. Instead, it's intended as an additive to Tamiya gloss paint, to make it semi-gloss or matt — the more you add, the matter the paint will become. I don't recall the recommended ratios, but I do know it's nowhere near 1:1. You can easily experiment with it to find a good ratio, though: take, say, five brushfuls of gloss paint and mix one brushful of Flat Base into it (giving you a 5:1 ratio), then paint a test swatch with it. Do the same with 4:1, 3:1, etc. and see which results you get.