[scorpio_rocks]

Its all about what can "take cover" in a trench/dug in and whether they can do it whilst moving.

from the game guide:

Fire Trenches provide cover (even if you moved) as for being dug in. Major obstacle to vehicles, which can be ditched trying to cross these, they are meant to represent wide fire trenches, not foxholes. Due to the ditching possibilities these can also be seem as a form of AT ditch. Ordinary foxholes protect only infantry, who will be counted as dug in if they start the move in such. The vehicle revetments and gun positions issued at the start of a battle can NOW be reoccupied. (WinSPMBT v102)
NB, Some forts and bunkers may have a carry capacity, and hence provide shelter for 'passengers'.

So the"emplacements/trenches" got by Digging in will ONLY provide cover to the correct troop type (Inf or Vehicle) and the unit will ONLY count as in cover if it started the turn in the cover.
A bought "Trench" will provide cover to all troop types as soon as they arrive (and it may cause vehicles to become stuck)