What distinguish fixed filters
They are not visible for users. Users can only see standard filters above the records on the table screen.
They cannot be overwritten. Fixed filters are always applied to records presented on table screen, even if a user explicitly set different standard filters. They do not allow for sorting. They only limit visibility of records. User can always apply own sorting pattern.
Note that fixed filters limits the visibility of records in the table view. They do not provide full and secure access limitation. To control user access you need to use row access control mechanism.