Secondary Targets When Attacking Groups/Mobs

Particularly with Ranged Talents like Hail Of Arrows, how does the Secondary Target momentum spend apply when attacking Squads or Mobs of monsters? As overkill damage on a particular member of the group transfers over to the next member, is the group the primary target and so secondary damage will transfer only to another group, or if just a particular member of that group the primary so if there’s overkill and the damage transfers to another member of the group but doesn’t kill it, the secondary damage can be applied to that member which will damage it further (and if killed, will then transfer over to yet another member)?

You’re targeting a specific member of the mob or squad when you attack, as you can specify which minion you’re targeting–once you deal with the blow-through damage and figure out who’s quick and who’s dead, you can pick another minion in the group as your secondary target, or (I would allow) a minion from another mob or squad that’s adjacent.

Yes, as @Maxspire described, you pick a single character in the mob as your primary target. This gets the initial damage applied, then the “spill over” damage is applied to the next minion in the mob - player’s or GM’s choice which one.
Then you decide to spend Momentum for the Secondary Target spend. If you chose to do so, then “A second target within Reach of the primary target is also affected by the attack, and suffers half the attack’s damage, rounding up (to a random hit location, if physical).”
This could be another Minion of the same Mob, or it could be a different character standing within reach of the primary target - maybe a Nemesis or Toughened character close to a mob guarding this more important character. The main condition is “within reach”. That applies usually to all members of the same mob, but you have to look at the actual positioning, if you want to affect a different character outside the mob.

Thanks, thats how I figured it was working, just wanted to consult the hive-mind to check I hadn’t missed anything.