MicrostockGroup

How do you indicate that images are exclusive to Alamy other than writing it in the description field?

Oh, of course - the option is greyed out for me because you can't reset license type once you've managed your images. Shame.
I'll look out for it next time I upload pics there.
They did contact me once to see if I could make an image available exclusively: I could, but it never came to a sale.

I think exclusive means exactly what it says. Once a buyer purchased it, the image will not be available anywhere else, for the time period when it's being used.

That's right. Just that it wasn't clear if you were using "exclusive" the way IS or DT use it, which is different. They claim the images are only for sale on their site. (which we've found isn't true because people sell similar's elsewhere and in one case, the guy claimed they were exclusive for higher commissions, on every Micro site he had them!)

And no I don't think it makes a difference on Alamy, because, if someone wants they can ask and request an exclusive use, even if it isn't marked as such. (opinion)

Maybe you should ask on the Alamy forum where more of the high producers who are tuned in to Alamy and bigger agencies, can answer?

Funny, I just 'managed' three images on Alamy and I didn't have the option for 'exclusive'. The only option I had was Rights Managed, which came up, greyed out, as soon as I put 'no release', so no option to choose Rights Managed - Exclusive.
I can see one benefit in setting the Exclusived there (assuming the photos is exclusively there) and that is that you might not be available if they wanted to contact you, so you could miss a sale. But I guess that doesn't happen very often. (It happened to me once (not via Alamy) when I was in the boonies for two weeks, mostly out of cellphone contact, and I lost a sale).