Slitherine

we know that normally tanks have a tag called "rott"
And tank destroyers have a tag called "fixetd"

how does these 2 things work in the game?

I believe if a fixed turret attacks a rotating turret, they suffer a -3 to Initiative. This is to encourage the use of tank destroyers as defensive units, or on the offensive against suppressed targets or with mass attack support.

But at a ~3km+ range of late game tank destroyers I find it hard to believe a fixed turret would be at such a disadvantage.