What are unknown BT clients?

I was looking at a peers list in utorrent and a user had a weird client called BTm16Btuga and that got me curious as to what kind of client it was and I did a search at google and it got me to a site about unknown BT clients.

Why are they considered unknown BT clients? Is it because they are in bete or just because no site hosts them and they aren't available to the public yet?

Unknown clients are ones that aren't recognised by the trackers of d/l sites, like utorrent which pisexy didn't recognise until about a month ago, it's usually clients that haven't become popular enough, like the one you'll find here; http://www.analogx.com/contents/news.htm

Stay away for them.