What's the difference between "-dm" and "-Dm" in GNU Screen?

The GNU Screen manual says:

`-d -m'
          Start `screen' in _detached mode. This creates a new session
          but doesn't attach to it. This is useful for system startup
          scripts.

    `-D -m'
          This also starts `screen' in _detached_ mode, but doesn't fork
          a new process. The command exits if the session terminates.

-dm is pretty clear to me: - screen forks a new process to run the provided command (or a shell if nothing was specified). - By "fork" it means that weird Schrödinger's system call in which the source code doesn't know if it's the parent or the child until the return value is observed. - And this new process is recognized by screen as something that can be attached. I noticed that -dm returns control of the shell, but -Dm blocks. So my question is: - Why does -Dm block? And how is that related to its lack of forking? - What does it do instead of forking? I think it still creates a new process, because "detached mode" suggests a process identifiable by a PID which can be attached. - What's the use case of -Dm instead of -dm? Thanks!