API is a one-time fee for developers. So if you are using a commercial program, you don't pay an API fee.
If you are using an off-the-shelve program that says it can use IQFeed as a data feed, like MultiCharts, AmiBroker, etc. there is no additional API fee. If you are the developer of MultiCharts, AmiBroker, etc., or your own platform, you pay a one-time fee. So if you are writing your own program to connect to the IQFeed servers, you need to register your software with DTN. For this, you get access to the developers kit; examples, documentation, help, and a DLL that is necessary to connect to IQFeed. That DLL is why IQFeed only works on Windows. If you are a software developer writing your own platform, IQFeed is a good choice IMO, I use it. My information may be out of date or just wrong, so check with IQFeed for the latest.