This fee is determined by how the client is using the photographs. Typically, the larger the audience for a specific use, the higher the usage or licensing fee. For example, a photograph used in a print and web ad campaign for a consumer product would have a much higher licensing fee than a photograph used in a business-to-business company brochure — the former has an audience in the millions, the latter perhaps a few thousand.
More Use = Higher Fee
Remember that this part of your overall price is not affected by how difficult the photograph is to execute or what the production expenses are. Those issues are calculated elsewhere. This fee is all about the use by the client. It is possible for a photograph, executed with minimal expenses or expertise, to generate an extremely high licensing fee. The point here is the use, not what it took to create the image.
A number of useful resources for pricing photography are listed on ASMP’s pricing guide page.