Royalties are typically calculated by quantity: you do N transactions, you pay N x Y to the rightsowner. For photos it could be the number of magazines you prin

Royalties are typically calculated by quantity: you do N transactions, you pay N x Y to the rightsowner. For photos it could be the number of magazines you print, or a percentage of sale on each magazine, or the number of impressions on a website, etc. "Royalty free licenses" are typically one-offs. You pay a fixed amount and then are free to reproduce the work as many times as you want. Some of these contracts might be limited, i.e. you can only do it in one magazine issue etc.