So what is the difference between 5HD and 7HD?

Minor addition, both 5Hd and 7Hd recoil from shooting if beaten (less than opponent Bw or Art, but more than half).

Surely the answer is 2HD?
😉

I was sooooooo tempted to say 2

7hd look cool
5hd are for wimps

Yep.

Page 12, last paragraph in the “Recoiling” section:-
        “A recoiling or pushed back element whose rear edge or rear corner meets terrain it cannot enter, a battlefield edge, friends it cannot
         pass through or push back, an enemy, or a city, fort or camp, (or a BUA gate – see FAQ) ends its move there (see figure 19b).
         An element already in such contact with any of these cannot recoil and is destroyed instead (see figure 20f).”

Interestingly, if a Horde scores less, but is not destroyed, they won’t recoil.
This means that any friends attacking that enemy in the flank or rear won’t recoil either.

Page 11, first paragraph of the “Combat Outcomes” section:-
        “A supporting element in close combat against an enemy element’s flank or rear recoils if the
         friendly element in combat with that enemy’s front recoils, flees or is destroyed (see figure 19c).”

I've generally thought of the difference as motivation: 5Hd are excited to be here, 7Hd would rather go home.

And of course they spring from the DBM/DBMM lists

5Hd represented either Hd(S) or Hd(F) whilst 7Hd represented Hd(O) or Hd(I)

In case the original poster is not fully across the whole DB(X) experience
S = Superior - adds 1 to the combat roll if they lose
I = Inferior - subtracts 1 from the combat roll if they lose
F = Fast - have a better movement rate and add 1 to the combat roll in their turn
O = Ordinary - no further modifications

When DBA2 was published and the lists were brought into line with the DBM lists, Hd was introduced as a troop type (It didn't exist in DBA1) - and so the 5Hd was repurposed as 5Wb

Come development of DBA3 and the creation of the Fast and Solid sub categories 5Wb became 5Hd

Cheers