It means it's not available as a sub.

Check the rest of your port and see how many of them are set like that. There was a brief time last year (when subs were first introduced) when some new images were marked that way, even if your default was to allow subs.

ETA: I just looked at your port (I should've done that first!) and see that it's a recent happening. Looks like your default got reset to "pay as you go only". I'm not sure where it is ... somewhere in control panel? ... but it's there, somewhere!

I can't find a default on the iStock website.

Maybe there isn't one. I use DeepMeta, and it seems to remember the last setting in the "Allow subs" Options box on the General tab of the details page of an image. At least, it's always check-marked for me.

Okay, I went through my portfolio, and found the the occasional ones that weren't checked for subscription. Weird. I never unchecked them in the first place. Anyway, thanks to all, for the explanation.

That's the point... Subscription was introduced last year and iStock didn't want to force any contributors to become part of the program. I think by default all images were not part of the subscription program unless explicitly opted in.