[RightDeve]

Since those FOs are forward deployed units they tend to receive more suppression and casualties than A0s, so sometimes their ability to call in arty is severed.
Maybe that's the case. Also experience plays the role here, as HQ units *sometimes* have higher experience than FO units.

FO can be seen as an extended eyes and ears for the arty battery at the rear, their duty is to spot the target and give that information *directly* back to its parent unit i.e the arty battery; not to just spot it and give that borg eye to A0.
So the actual order of firing arty in RL (roughly) would be: any HQ unit (be it platoon HQ, coy, or bat) orders the shelling of a specified target to the FO, then the FO spots it and send firing order (adjustment, number of shells, etc) directly to the arty guns. HQ may intervene to stop the firing or order to fire next target.
In fact, some PBeM house rules at theblitz for example prohibit the use of non-FO units to call in arty to reflect the real FO application in (most) RL situation.

I think SP system is flexible enough in allowing both HQ units and FO units to call arty, because there are (many) occasions in RL where HQ is the sole unit able to call arty.
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To the original question: Yes, FO and A0 (more properly HQ) are separate unit classes. But both have the ability to call in arty.


Regards