[gila]

I may be wrong but here goes.

A forward observer is just that,he reports enemy locations back to the AO. which then directs arty calls,with a better chance of hitting something.

Although fo's can plot arty,just not as good as the AO and slower in most cases,

Some arty formations have thier own Obs,then best to use them,if they have eyeballs on target,i assume.