Group Members API - Inference of different values of type attribute in User object in payload

[ShwetaGulati]

In API
https://developer.atlassian.com/cloud/confluence/rest/api-group-group/#api-api-group-member-get

What does these 4 values imply in type field of User object?
When will be user’s type:

1. user
2. known
3. unknown
4. anonymous