MFF2 (Machine Form Factor 2)
Updated
MFF2 is a SIM in a small chip package soldered onto the device’s circuit board instead of a removable card. It resists vibration, temperature and corrosion better than a card in a slot, which suits vehicles and outdoor equipment.
In practice
An MFF2 SIM cannot be swapped by hand, so it is usually an eUICC whose operator profile can be changed remotely.