What's the deal with asymmetrical Crossfire?

[manveruppd]

Just curious, as I'm finally getting a board that can support X-fire. I've seen asymmetrical crossfire demonstrated in reviews, but is it something that all Crossfire-capable motherboards can do, or is it only specific ones? And does any pair of Radeon cards work, or only specific combinations?

 

[Surveyor]

Isn't asymmetrical CrossFire a reference to different GPU's being used together?

To some extent you can but the options are limited as shown here:

 

[manveruppd]

Yeah that's what I meant, although it doesn't look all that asymmetrical at all, seems you can only go 1-2 steps up or down.