It’s a system table. You can find it under AOT > System documentation > Tables, nevertheless you’re not allowed to change the schema of system tables. But you can modify SysUserInfo table. Or create a new mapping table.
Sorry, I don’t know how to create a new Mapping. If I create a new Mapping table, will it help me to input the Department Name of each user through the SysUserInfoPage form and also to sort/filter on this field?
The mapping table would contain references to primary keys of tables being mapped (user ID and department ID in this case). You surely can join such a tables with other tables for filtering or any other purpose.