Entity openers open action inventories based on the entity a player used or attacked.

They look like:

{
  ...common fields...
  "entitySelector": "",
  "entityPredicate": {}
}

(common fields are specified in Openers v3)

This is an entity selector that clicked entities are matched against. This is a string.
See here for information about entity selectors. "@s" is stuck on the front of what you provide; so, for example, "entitySelector": "[name=Bob]" would go to "entitySelector": "@s[name=Bob]". You can also use this for creating selectors. Just remove the @whatever from the front.

This is an entity predicate that clicked entities must match. This is an object or array.
See here for general info on predicates. You can also go here for creating them.

Entities must match both the selector and the predicate.