ROF and ACC for guns

[panzer51]

I see some ground elements have ROF and ACC added to them. So I assume this is Rate of Fire and accuracy. But what exactly these numbers mean? For example, 150mm sFH18 Howitzer has ACC 650. Does it make it more accurate?

[Denniss]

devices have basic RoF and Acc, ground elements may have further modifiers (positive or negative) that are cumulative.
So positive acc will add to basic acc, negative RoF will substact from basic RoF. More Acc = better

[panzer51]

What's the reason we have these modifiers?

[Denniss]

device is basically just the gun barrel, mods from element take other things into account like optics (+ acc) or cramped conditions in tank turrets (-rof) so you may use the same device in different ground elements with different multipliers.
Example the towed 75mm Pak40 AT gun and the variants of it mounted on Marder II/III and other vehicles