1-6 is hotbar. X switches to your other hotbar, which is also 1-6. R is mining, N is scanning, I have no clue what painting or wiring are.
Wait, there's another hotbar? I feel really silly for missing that.
Wow, seriously, this is actually a useful feature?? I thought having 6 slots was weirdly limiting, but with a second hotbar it's actually not too bad -- everything's pretty reachable with just one hand. Was I just completely blind or is this feature not advertised anywhere?
It is supposedly mentioned at some point in the tutorial/introductory mission, by an NPC... which yeah, basically means nobody ever read it and only stumbled on it because they knew X did something before.
The thing with an action-game interface, is that gating any part of it behind a toggle switch, implies that at any given time, the locked-off part is not supposed to be used, which is all kinds of silly for a "quick"-use interface here. They should have made the second half accessible via 7 through = keys, at least.
Speaking of which, hypothetical design for a better, super-hybrid hotbar:
Slots expand or contract depending on whether they have a two-handed item, a one-handed item, or two one-handed items in them. The button in the middle switches the sides of the bar, for the same basic effect as the new bar and the X switch. The lil' button on the far left switches left/right hand items in the current slot.