After you close a user account, do not reuse that account's user ID number. It is possible that files somewhere on a system could still be owned by the ID number, or files may be placed on the system from an old backup tape. These sorts of file would be compromised by associating a new user with an old ID number. In general, the rule is that the ID number is the permanent property of the user to whom it is assigned. For more information on user ID numbers, see the passwd(4) reference page.