Tanks will automatically turn to face threats if given no orders. I tested the details of this behaviour with nande. The results:
Tanks only turn toward spotted targets. It doesn’t matter if the tank itself has line of sight, another unit can be doing the spotting, but enemy units have to be spotted to trigger automatic facing behavior. [edit: nande believes that line of sight is mandatory for ground threats and is only ignored against helos, and he’s probably right; An open question is, do helos ignore line of sight against ground targets when determining their own facing?]
Only units within a certain range are considered. A hind was ignored by a leclerc if moved beyond 2500m. It could have either been using the range of the weapons of the hind (fleyta, 2450m), or it’s a global ~2500m for all units.
Which threat to turn to is determined by the weapon types of threats. A hind with a fleyta was considered more important than a tank (chonma V) at point blank range and more important than a lie ren squad (20AP). Varying range or introduction order of threats didn’t change anything, and the hind was respected even with weapons disabled and no ammo on the ATGM. So ATGM always trumps infantry RPGs and tank cannons. Rocket pod helos were ignored in the presence of tank threats.
Of course the tank will try to turn halfway and show its front armor to all enemies if possible, but if the threats come from too different directions that can’t be covered simultaneously, it will prioritize protecting itself from ATGM. Didn’t test what happens if there are multiple ATGM threats.
It might be viable on some maps to abuse this behavior by sending 20pt atgm jeeps to the side, so that enemy tanks will expose their sides in an attempt to face them. The challenge is in making sure that the jeep is spotted yet still alive.
EDIT: FLX said that unit facing is decided in code based on DPS. He was speaking off the cuff and from memory and I’m not 100% convinced, but the mystery deepens. A theory of mine is that the puissance weapon stat (visible with mod tools) could be what decides the threat factor. But I don’t care enough to test.