/assist [target=Focus]
/cast Whatever

VS.

/cast [target=FocusTarget]
/cast Whatever

The difference between these, is if you want your toon to actually target whatever your Focus is targeting, or if just casting at that target (without actually changing targets) is enough.

The full syntax would be:
/cast [target=FocusTarget,Help][target=FocusTargetTarget,Help][Target=Focus] Heal

a) A friendly targeted by your Focus.
b) A friendly who it targeted by the target of your Focus.
c) The Focus.

But your example got the point across.