Hi Jonas,
Great question!
This is actually something we are currently reviewing, but right now we would generally ask that you buy the appropriate license according to the number of sites you will be using our plugins on.
So, if that is just one site (that happens to be part of a multisite network) a personal license is fine – but if you will be deploying our plugins on multiple sites – whether independent or part of a network – a developer or business license should be used.
With regards to dev sites, we generally don’t require or expect you to have a license in place (though if you wish to receive and test automated updates in your dev environment a license key would then be required – generally that’s not something most people need however).
Does that answer your questions?