The primary IBM i user object is called a "user profile". These objects correspond pretty closely to other system's user ids or user accounts. They are not organized in a directory like AD, so we don't call it a "user directory".
To make matters more confusing there -is- a different "user directory" called the "system directory" or just the "directory" that contains additional user information used by a variety of tools like SNADS, and IBM i based email. It is managed separately from the user profile.
IBM i provides a hierarchical file system called the IFS (Integrated File System), and users can have "home" directory folders in the IFS: commonly called the "user directory" or "home directory".
Finally, IBM i also provides an LDAP directory which can also contain user information - one example when using EIM for SSO.
So when you say "user directory" without providing any context, is really isn't that clear what you're referring to.
-- DaveSlash