He's referring to "similars". Most RM agencies don't want you to upload similar images as RF and RM. Microstock agencies are the same way - if you have an exclusive image at DT, or submit an image that gets accepted to their contests that require exclusivity, then you aren't allowed to upload "similar" images elsewhere.

The term "similar" can have various subjective meanings. I've had a Getty photographer tell me that she can only upload 3 images from a shoot at a time to Getty. Not sure how true that is, but if you follow the RM forums, you'll hear a lot of the old school photographers bashing "amateurs" for submitting too many similar images. That's also one of the reasons behind AlamyRank at Alamy.

Funny thing is, you've got folks like Ron Chapple who have been doing this for most of their lives, and have even started their own agencies, encouraging new photographers to shoot similar images so the buyer has a choice.

I know what I'm doing...you can decide what's best for your business