Counterbattery will fire upon:
Nebels
Walking Stukas
Stugs
Volksgrenadiers
Snipers
WM Mortars
PaKs
88s
Tank Buster Grens
All PE HTs
The Nashorn
Hummels
Hotchkisses on occasion
Hetzers
Any of those units firing on an opposing (allied) player's units for over 5 seconds will come under a 5 shell barrage with ~20 second cooldown. No range afaik, and the ability targeting the not-so-obvious stuff (volks? wtf?) is definately a bug.