Rights managed is the golden goose of stock photography but it is not easy to get into a rights managed agency. Most photographers who submit to stock photography agencies may never get the chance to submit to a rights managed agency. The competition is fierce because this is where the money is. Rights managed means that the agency will negotiate the sale of your photo to the buyer based on a number of factors. These factors include, but are not limited to, the industry of the buyer, what use the photo will serve, and how much money the buyer can pay. Normally the buyer asks for certain restraints to be put on the photo after purchase. One example might be that for the next three years the photo is not allowed to be sold to any other buyer looking to use the photo for an advertising campaign in America, or another example would be that the photo is not allowed to be sold to an Australian company in the oil and gas industry for the next year. Each contract is custom made and is typically completely handled by the agency, not the photographer.
No matter what kind of stock photography you are going to sell, it is important to understand your rights as a photographer and what rights you are giving to a perspective buyer.