OK, here’s some confusion about licenses: I was under the impression that I buy a license for a certain client, irrespective of who is responsible for a website eventually. If I’m subcontracted by a “smallish” agency to create a website using Kirby, and that agency itself is selling its services, including that website, to other companies, how would I know which license to register? I thought the license would count for the direct client I work for?
No, the license is for the website and hence for the owner of the website, not any intermediaries. Your best bet is not to buy the license yourself, but let the end-customer buy it or the agency whose customer this website owner is.
If you still want to buy it (which is probably not really a good idea anyway in this contellation), then this end-customer needs to tell you or the agency (and this agency then tell you) which license they need.