What does the "x" character in the second field of the user account record of the /etc/passwd file indicate?

When the second field of an /etc/passwd entry contains 'x', it signals to the system that the actual password hash is stored in /etc/shadow rather than in /etc/passwd itself. This shadow password mechanism restricts access to password hashes since /etc/shadow is only readable by root, unlike /etc/passwd which is world-readable, thereby improving security.

Sharing an account among multiple users is not indicated by any character in the password field - Linux user accounts are identified by UID and there is no such field marker for shared accounts.

A disabled account is typically indicated by '!' or '*' in the password hash field of /etc/shadow, not by 'x' in the /etc/passwd password field.

A non-existent account would simply not have an entry in /etc/passwd at all - the presence of any record in the file means the account has been defined on the system.