Let's imagine an application where users can work collaboratively on projects. There's a project listing where a user can subscribe to a project, and a coworking screen where multiple users can work on a project.
So far I have identified the core domain, "coworking", and a subdomain, "collaboration project listing".
Both project and user need some sort of backend, where user settings can be edited and projects can be created/edited. I haven't really got a clue, so I identified this as a subdomain "backend", which seems too abstract, because it doesn't describe any sort of business domain. Data from backend is needed by both domains that I mentioned above, so I'm hesitant to attach the backend to either of those.
Is it legitimate to have a "backend" subdomain or is it an indication that my domains are not clearly defined?