What are "Horde summons"?

[Scarlet E.]

"Elemental Surge (Lightning)" ability's description is:

Transform damage of summons into lightning.
Gives a 10% chance to electrify on hit.
Horde Summons are excluded.

However... I think that this skill itself is the ONLY place in the game that I encoiuntered "Horde Summons" term.
I don't know, maybe just I missed it during my 40h of dameplay, but I honestly have no idea what can that mean.

Are Astronomer's Orbs a Horde Summon?
What about the Blades? Or Arcane Splinters?

Any idea which Abilities this skill can be relating to? :-(

[Sire]

I'm assuming Horde Summons refer to that on the Rings (Necromancer Clutch, Demonic Bond). So, those summoned units are not affected by Elemental Surge.

Summon has its own tag for abilities. Orbs, Golem, and Spirit Warriors are considered Summons, and will benefit from Summon effects.
-- Also, I think the Warlock's Main Weapon technically counts as a summon as well for some reason, although I haven't hard confirmed that.

[Finvaara]

Horde Summons are defined in the sidebar when you're thinking of selecting that trait. It's currently just the rings and the warlock's main attack, I think, but the orbs might also count. It says items and abilities that continuously call forth summons into a group.