Category:Sort key tracking
These categories are added by the
format_categories function in Module:utilities, if a sort key is provided to a template (the sort= parameter). The split between the two categories is determined by the sort key that is automatically generated. If it matches the provided sort key, then the provided sort key is redundant. If they don't match, the sort key is needed.
Ideally, the "redundant" category should be empty altogether. The "needed" category should contain as few entries as possible, and sort keys should be added to Module:languages to that effect.