At deployment always set ATGMs, may sometimes set troops less often armour tend to set these if needed. When selected gives yellow text so you know its on.
ATGMs-
Set infantry soft ranges very low, if its a good ATGM can set min armour so does not engage APCs. Sometimes this is your most powerful armour killer penetration wise.
Infantry sometimes adjust armour range, 2 reasons.
1) RPGs with range of 500m often set to 300m or so as more acurate. means dont give away position & can decide in your turn to take longer consecutive shots.
Adjust individually for ambushes if need.
2) If they have GL with a longer range than RPGs often restrict armour range to that of the RPG.
Armour-
Depending on map may restrict range vs infantry based on their RPG range to stop firing at none threats to it.
Can set min armour for some so ignore APCs, do not do this for all always need at least something firing at transport.
Very rarely restrict max armour I would rather everything bang away at them even if cant kill, may get lucky & can suppress giving a greater chance they miss.
Hardly ever use overwatch hex but on the odd occasion it can be usefull if you have an idea whats out there. Let it fire at infantry in that area if you know a MG is there for instance.
Main use is ATGMs & ambush or confined space defence to stop distraction buy other units.
Open ground the standard routines work very well anyway.
The other use is if helos about decide if its worth switching AA off on units in the hopes of not giving position away or to stop shots being drained by helos.