denote that the slot will accept different card thicknesses:-
1.A Type I slot can hold one Type I card (which is up to 3.3mm thick). This slot size is normally used for RAM or ROM cards.
2. A Type II slot can hold one Type II card or one Type I card (which is up to 5.5mm thick).
3. A Type III slot can hold one Type III card (which is up to 10.5mm thick) or any combination of two Type I or II cards.
The modem you've seen will almost certainly be a slot II size, so will fit both a type II and a type III slot.