External switches work great but there are some limitations.
They control one MIDI CC (onboard switches can control two, plus custom MIDI).
They can't switch presets (no MIDI PC).
You need to use the parameters TyHDW and TypTog to configure them.
This is different for momentary resp. toggle/latching switches.
Latching:
When set to TypHDW, the switch state (on/off) is carried over when switching preset, because the MFC-101 polls the switch after having switched presets. This is okay for some situations. But it interferes with the MFC's "Axe-Fx Mode" concept, where the preset determines what's on/off.
You can set the switch to TypTOG instead, which prevents polling, but this can cause the external switch to lose sync, sometimes requiring two presses to get the switch synced again.
Momentary:
when set to TypHDW, the switch operates as a momentary switch, so retaining states when switching presets is irrelevant.
This won't work for stuff as X/Y-switching.
That's why you can set them to TypTOG, which will make them act like latching (toggle) switches. But you won't have LEDs indicating their state.
And the sync issue may occur here too (I don't remember).