You would have a D-plate signal just before the transition from non-signaled to signaled territory. This could be approaching an interlocking (crossing) on an otherwise non-signaled line or where you are going from track warrant to CTC territory.

For example, you would have the D-plate signal while still within track warrant territory. It would not provide any information about the occupancy of the track segment ahead -- not really a "block" in the CTC sense -- and you would continue to operate on track warrant authority as you pass the signal. However, the signal would provide information about the first CTC signal a mile or so ahead.

That lets the crew know whether to expect to stop at the first CTC signal or to be able to pass it.

Hope that makes sense.

-- Ernest