Was perusing some old test template files - they worked way back when but are disassociated now so don’t - and came across this within a weblog:entries tag—{segment_3_category_id}
I usually comment my code pretty well but the only note close to this was the weblog tag was getting variables passed from another template. Segment_3 would have been the url_title of an entry in this case.
Can’t say I recall seeing this before and a search didn’t really get me very far.
Any ideas/enlightenment would be greatly appreciated.