It is hard to speak generally about [AF1] and [AF2] buttons - it is better to speak about a specific VOICE or situation.

The AF buttons are Assignable Function buttons - which from its name you can tell can do different things in different Voices. In one Voice it might bring in a different sound, or it might add an effect or it might change a bowed string into a pizzicato string pluck... it is "Assignable".

The behavior of the button can be "Momentary" (stays on only while you hold it) or it can be "Latch" like a light switch it stays On until you turn it Off. This is programmable on a per VOICE basis. It depends on what you want it to do.

Let's look at a few examples:
the VOICE: "Vintage'74" - an emulation of a 1974 Fender/Rhodes with rubber hammers...
The AF1 and AF2 buttons will "Latch" when pressed.
AF1 will stop the AUTO PAN L/R movement. They are assigned to control one of the INSERT EFFECT parameters.
AF2 will bring in a Pad Element that layers with the Electric Piano. This is assigned within the MOXF's XA CONTROL (Expanded Articulation Control) function... where you can assign an Element to play only when an Assignable Function button is pressed.

To see where these are assigned:
Press [EDIT]
Press [COMMON]
Press [F4] CTL SET (Controller Setup)
Press [SF1] Set 1/2

Here you can see on the right half of the screen where AF1 is assigned to control "INSA: Ef PanLR" (Insert Effect A, Effect Pan Left/Right). the DEPTH is set to -20, so pressing AF1 will reduce the Left to Right panning motion.

To see where the XA CONTROL is assigned...
Press [1]
In the screen you should see OSC/ WAVE screen
the numbered buttons on the right front panel are now ELEMENT SELECT buttons 1-8. This particular VOICE has six Elements (you see that the bottom row of buttons has six buttons lit). Buttons 1-8 SELECT the Element, and buttons 9-16 MUTE/UNMUTE the Element.

If you press each button 1 through 6, in turn you can view the ELEMENT 1, 2, 3, 4, 5 and 6 on the OSC WAV screen.
You will notice that Element 6 says "XACtrl = AF2 On" meaning it will only make SOUND when AF2 is lit. ... This Element is using a Waveform called "Dark Light" which is a nice smooth pad sound that layers nicely with the electric piano.

While still in EDIT, press the [COMMON] button again
Press [F1] GENERAL
Press [SF4] OTHER
Here is where the Assignable Function 1 and 2 buttons can be set to either "Latch" or "Momentary"

Example 2:
Call up the VOICE: "Steel Slide Vel"
Here the AF1 and AF2 are momentary and they bring in a different articulation for the guitar sound. You would hold it, play a note, and then release it to return to normal.

Exploring the ELEMENTS here will reveal that the XA CONTROL which only allows Elements to sound under certain conditions are programmed as follows:
ELEMENT 1 only sounds when "all AF Off" _ This is the guitar Open String sound
ELEMENT 2 only sounds when "all AF Off" _ This is the guitar Slide (velocity above 121)
ELEMENT 3 only sounds when "AF1 On" _ This is the mute string sound
ELEMENT 4 only sounds when "AF2 On" _ This is the Harmonic
ELEMENT 5 only sounds when you release a key "Key Off Sound" _ A "dead note" thud
ELEMENT 6 plays and sounds normally at all times "Normal" but only when you play above C6 on the keyboard (sound effects)
ELEMENT 7 only sound when you release a key "Key Off Sound" _ Those finger noise effects
ELEMENT 8 only sound when you release a key "Key Off Sound" _ Those finger noise effects

As you play this VOICE you can hear how AF1 is responsible for a MUTE tone
As you play you can hear how AF2 is responsible for the HARMONIC

All the different KEY OFF noises are set so that only when you play within a certain range and at a particular VELOCITY range will you hear a finger zing or scrape - these occur while playing the acoustic instrument and they occur here without you really having to do much other than play the keys - but if you listen closely you will hear a random finger noise or zing or scrape as you perform.

Velocity is the pitch slide (when you exceed a velocity of 121- then and only then will Element 2 sound)

So while this VOICE is 8 ELEMENTS no more than one Element is sounding at a time... XA CONTROL, your velocity, and the key mapping mean that when you play this VOICE you have a variety of different guitar articulations at your finger tips... you can mute a note, you can slide to the pitch, you can play harmonic, and the noise effects that come in as you play just lend a touch of realism that cannot be denied!

Hope that helps.