Just starting out here I currently don't have the rep-points to upvote his post, but I think that N3CAN is exactly correct. Regrettably the so called HotKeys on the Anytone 878 and 578 just seem to be something like "Links" or shortcuts to submenus, which then can be used as an assigment to one of the function keys, so the name is a little misleading.
I just set up my new 878 and in comparison to the 578 you have less programmable keys available. It is just PF1 and PF2 which are the 2 keys below the PTT, PF3 which is the blue key on top, P1 and P2 which are on front.
Now you can assign functions to short and long key presses of these keys which should make 10 functions programmable. But a few of the functions (like "Monitor" or "Tone Burst") occupy both, the "short-press" and also the "long-press" slot so you lose one "programming slot".
I just managed to assign 8 of them. It sounds like a lot, but it's not as you also have to setup basic functions like: "VFO/Memory", "A/B", "Monitor", "Select TX Power", "Scan", "Reverse", "Send Toneburst", "Select DMR Timeslot" and so on.
You quickly run out of keys to assign to. For example the radio offers "Nuisance delete" for scan mode, which i find very important, but currently I got no key left to assign it to.
Too long, didn't read: I think N3CANs answer is correct and it's a shame because this radio could make good use of a few more programmable function keys.
PS: There's also the Function Key (long press of the green key) and 0-9 key assignment, but it let's you only assign menu shortcuts and no other functions, so that doesn't help either.
If anybody knows more, any help is appreciated, thanks.