According to the link you provided, you are dealing with royalty free, exclusive, and non-exclusive licenses. Royalty free basically means that you will receive a set rate for your image. Multiple people can buy the image, and the buyers can use that image multiple times, but you will only get paid once. Now I'm not familiar with Alamy, but I assume they will pay you per image sale. This is normally the least profitable per sale, but you will probably have the potential to make the most sales this way.
The difference with licenses is that you can specify what your image is to be used for and set a duration time for the license. If the buyer wants to use the image for more than one purpose, then you can make them buy another license. Also, once the license expires they will have to buy another one if they wish to continue using the image. Non-exclusive means you can sell the same license to multiple people. Exclusive means that once you sell that license, you can't sell it to anyone else until the buyers license expires because they paid extra for "exclusivity", meaning only they can use the image.
Licenses will grant you more profit per sale, but will probably turn people away who would rather save money and buy a cheaper royalty free image that they can use over and over again without having to adhere to the restrictions of a license.
Hope that's what you were looking for.