Subprojects are an account management tool for grouping resources together.
Subprojects cannot be directly linked to a billing account. Linking happens through its parent project. And they cannot have other subprojects attached. Only parent projects (a.k.a. projects) can have subprojects.
As you can imagine, subprojects are useful when acting as an application service provider (ASP) for Sinch Products. The ASP can create a subproject for each new customer and simply remove the subproject if the customer leaves. Usage for each customer is measured separately.
A unique API key can be generated for each subproject in the customer dashboard, but it is also possible to use the key of the parent project to access subproject resources. Once a subproject is created, it is identified with a subprojectId.