[Imp]

90% of the time there is no need to use it the standard game filtering is pretty good..
Exeption is I nearly always try to buy ATGMs seperatly as a section & set for them.
Reason not good vs soft targets & only a few shots.
So one slot is set
With vs soft range 4 vs infantry range 3.
Other uses
You only really need to do if the situation arises dont go mad setting everyone.
Infantry in ambush with long range RPGs set armour engage range to 4-6 instead of 10 to increase hit chances if need to.
Armour sometimes usefull in open like dessert to restrict infantry engage range so they dont get distracted.
If short on stuff that can kill his can set minimum armour so ignore APCs remembering to leave someone to shoot at them.
If your opponent has gone helo mad switch off AA fire on some tanks so they do not give position away.

Basically set for ATGMs at start & for the odd unit as needed to suite the situation.
If you are playing someone who uses silly tactics like running jeeps APCs helos round to draw fire this is when you set it & give them a nasty surprise.
Several people over use setting everyone, this often works in your favour letting you do silly things so dont as not as flexible as standard routines.
Its very powerful if used right but can get you in trouble if abused, bonus over using the Y key to set ranges is unit is highlighted (text in yellow I think) so can remember to switch back if want to.