It's just the units that were hit by the sub. What it means it that if you have a destroyer any units taken as casualties in opening fire from sub hits still get to fire back.
Yeah, what Ted said.
I think the rule book states that the presence on destroyer units give naval units warning and thus they can return fire instead of being eliminated before your fire phase.
This rule only applies to attacking subs, any defender hit during the AA phase or Bombardment (including destroyers with the technology) never get to return fire.