While @r alone can only target player entities, adding the type parameter allows it to target other entity types:
/say @r[type=ArmorStand]But because of this requirement, you'd only be able to target all except for a specific entity:
/say @r[type=!Player]Attempting to use an invalid ID will throw an error, stating "Entity type [name] is invalid", rather than targeting all except that invalid type.
/say @r[type=!InvalidID]How would I cause @r to target all possible entities without excluding any?