Over categorization is the concept (on Commons) of having an image both in a parent and child category (see https://commons.wikimedia.org/wiki/Commons:Categories#Over-categorization). So for example both Category:Europe and Category:Netherlands.
As a bot operator I want to provide a list of Commons categories to a service and the service would return a list of categories without the over categorized categories.
I used to run such a service on Toolserver, but it died with the toolserver. The ErfgoedBot (heritage for Wiki Loves Monuments) categorization bot and the general categorization bots on Commons would hugely benefit from this service.
See T57644 for an example where this came up.