The menu for selection mode and filters (remain selected) is a bit confusing, indeed. However, selection mode and filter in that menu are two separate things.
Selection mode (squad, platoon, coy) is not a filter. It defines a type of automatic selection when you drag a selection box around one or more units.
- Selection by squads: "standard" selection, i.e. drag a selection box and those units are selected
- Selection by platoons:
drag a selection box around unit(s) and all the units belonging to the same platoon(s) will also be selected in addition to selected unit(s).
- Selection by coys: works like selection by platoons, but this time selects companies.
Filter (
remain selected), if applied, defines what types of units will remain selected - as discussed here.
Hopefully this was helpful.