I'm not sure what parameters are involved but guessing isolated objects that have partly transparent elements that can't easily be extracted with a magic wand.  I feel a whole lot of exclusive images involving glass and ladies with see though clothing coming on 

Funny no announcement unlike IS who announced and never delivered....

I just received their April newsletter in my inbox this morning. Why on earth wouldn't they have pointed this out?

I have some files I might consider offering a PNG for (I've never, and would never, offer RAW so I've never uploaded an alternate format). Some things with complicated masks - not clipping path sort of stuff - could have some real value if we could share the mask (and a separate JPEG or a PNG would be fine for that).