What is the purpose of nameidentifier claim?

Question

What the claim of type http://schemas.xmlsoap.org/ws/2005/05/identity/claims/nameidentifier should be used for?

This is the main question, and here are

Answer 1

Per The Role of Claims,

Name The unique name of the user

Name Identifier The SAML name identifier of the user

These two claims are part of the group of claims that AD FS 2.0 configures by default.

This implies that they are IP scoped.

e.g. when you log in to Google using ACS, "nameidentifier" is the unique GUID associated with your account by Google whereas name is your Google login e.g. "tim.smith@gmail.com".