What is the difference
 

If you are on a Defend mission you can entrench all of your units. You can also buy Trenches. In either your units show up as dug in.
What is the difference between them?
Why would you buy trenches if you can entrench for free?

Re: What is the difference
 

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)

Re: What is the difference
 

Thanks.
So would it be a reasonable tactic to build a line of trenches behind your original line with dug in infantry so you can fall back to them as the battle progresses?

Re: What is the difference
 

If you think its a good idea, then try it.

Re: What is the difference
 

I have done that a few times, keep in mind with your trench that the enemy can take take cover in your abandoned positions so don't let them or fall well back under cover of smoke. Good place for an arty gold spot if you think you will have to fall back.

Taking over enemy foxholes is a handy tactic sometimes when assaulting.