I noticed that on some unit cards there are normal surge symbols and there are surge symbols on large dots for units' abilities.
I can't find any reference for those in both rulebooks - is there any difference between those two types of surge abilities?

Those ones with circles are called unique surges. You can read about them in the first Latari Elves article. It is a new mechanic, so you won't see it in the rule books. We aren't 100% sure how they'll work, but the most prominent prediction is that you can only use unique surge abilities once per attack.

49 minutes ago, Embir82 said:

I noticed that on some unit cards there are normal surge symbols and there are surge symbols on large dots for units' abilities.
I can't find any reference for those in both rulebooks - is there any difference between those two types of surge abilities?

The preview article for the elf unit that those first showed up in said that they are 'unique' surges. It looks like you can only use them once per attack. If you play Descent or Imperial Assault, the unique surges appear to work like surges in those games do.