Member Ids and MemberLookupIds

Every members gets a unique auto-generated Internal Member-Id when created. This is system-wide and cannot be modified.
This Member-id is used is all database tables referencing the Member, and it provides the fastest way of identifying a Member is the platform.

However, the Member can be identified in several other ways. Wherever the API documentation says memberLookupId, you can pass in any of the following:

• The internal generated Member Id
• The email address. This will look up against the email field on the Member object.
• The mobile (cell) phone number. This has to be a full length number starting with + and country code
• The External Id, prefixed by $ (dollar), provided an External Id has been assigned to the Member.

Note that if the application calling the API knows the Internal Member Id then it's best to use this in the lookup since it generally gives better performance than if the member needs to be looked up based on say the email address.