This primarily refers to the phone interface!

On the Channelstrip, Mixer and Transport screens you will find blocks of buttons that can 'impersonate' other buttons. These are called "F-Keys" in the menus etc.

By default these buttons represent a block of controls on a hardware MCU that is labelled 'Function Select' (F1 - F8):

If you do not assign anything else to them, they will send the same notes that these MCU buttons would send.

Some DAWs have fixed function assignments for these buttons (and relabel them accordingly), some do not.

In DAWs that treat these buttons as 'user-assignable' you will usually find some kind of UI inside the DAW where you can define what action those keys will trigger. Below is an example from Cubase's Mackie Control properties window:
TouchDAW will also let you assign some other function to these buttons, or rather it will let them act as another MCU button:

Select "Assign F-Keys" from the menu and touch the button whose functionality you want to change.

You will then be presented with a list of all the functions that your DAW has assigned to some button on a MCU. Select what you want and the given button will change to call the selected function instead of the original 'Function Select F...'.

That way you can set things up so that whatever you need most frequently is directly available. Should you miss a record button on the mixer screen for example, you could create plenty of record buttons here.

Once done touch the menu again or double-tap the background to exit assignment mode.
Besides other MCU functions you can also make these F-Keys act as shortcuts to other screens in the app.

If your DAW treats the MCU F-Keys as user-assignable you can of course also mix things and for example assign functions to F1 and F2 in the DAW, while changing F3 and F4 in the app.

Key assignments are stored per screen and per DAW. That is: You can have different assignments for the F-Keys on the mixer and transport screens and you can have different assignments on the mixer screen for Cubase and Pro Tools (or whatever you are using). Storage always includes all available F-Keys of one DAW/Screen combination, though. If your DAW relabels the 'Function Select' buttons then once you have changed the assignment for one button, others that do not have an assignment yet may no longer read "F1", but show the DAW's default label for that button.

(Cubase note: After changing key assignments here you may need to click the "Reset" button at the bottom of the window for Cubase to actually activate new assignments)

Tablet Interface

On tablets the 'F-Keys' are found to the left of the 'Page' and 'Bank' buttons (Your DAW may have different labels for them).

There is no reassignment option in the tablet interface, because all functions that could be assigned to these buttons are already available on some other button, but they can of course equally be used for DAW-side assignments if supported by the sequencer.