It was possibly added for software list items. It was added by dankan1890 in 25bc1007af6054e083d5c671f93cbe94878ef0f8 (along with a whole lot of other unrelated changes) but never hooked up to anything. Lua scripts may be able to use it though. What is the covers collection anyway? I'm not the most knowledgeable on all these non-core resources.