https://github.com/lnurl/luds/blob/luds/04.md#wallet-to-service-interaction-flow outlines an optional action enum:

action enums meaning:

• register: service will create a new account linked to user's linkingKey.
• login: service will login user to an existing account linked to user's linkingKey.
• link: service will link a user provided linkingKey to user's existing account (if account was not originally created using lnurl-auth).
  auth: some stateless action which does not require logging in (or possibly even prior registration) will be granted.

This may be useful to distinguish for clients using js-lnurl