If you are working on a single application, you should choose this plan. You can start building your app for free and then when needing more features like a custom domain, you can choose a plan that suits your needs. By default, there can only be one collaborator per app on the free plan. If you need more collaborators on your app, you can choose a premium plan and add the number of collaborators that you need. An app with an "plan per app" can be transferred later in "team pricing" if needed.