How to ensure that migrations are always done from a specific source database?
Looking at the workflow setup, the step named “Create Migration Set” is typically set with a source database and thus only gets marked complete when the developer successfully creates a Migration Set from that particular source database. So you can be sure that the workflow will not advance to the later migration steps without that requirement being met.
Next, the migration steps themselves have an optional source in the setup, which locks it into migrating only from a Migration Set created from a particular source. The source database value is often left blank allowing more flexibility, but can be entered to provide this tighter control.