Whenever a Member Id can be specified, either as query parameter or as part of the url path, you can pass in any of the Member identifiers:

Member Id. NEON’s internal Id. Use this parameter where possible since it’s generally the most efficient way of identifying a Member.
Member External Id. Aways needs to be prefixed with the ‘$’ sign (This is to differentiate it from the other lookup types).
Email address. Needs to be a valid email address format. It’s not case sensitive, and always stored and returned in lowercase.
Mobile phone number. Must be the full international phone number format, starting with the ‘+’.

Note that email addresses and mobile phones may change over time, so if you want to make sure you’re always accessing the same Member then it’s safer to use the Member Id or External Id.

If Email address or Mobile phone number is not configured as unique then an error is returned if more than one Members were found. In such a case you need to use one of the other identifiers.

Sometimes the documentation states that you can pass in a list of Member Ids in the query parameter. In those case you can enter more than one Member Lookup Id, separated by comma. Note that when passing in more than one identifier, all of them need to be of the same type, for example a@domain.com,b@somethingelse.net.