Think of it as different levels of preparation.
Diagram Managers are longer-term - ensuring that train crew diagrams work, trains are fully covered. These diagrams need to fit in with the levels of establishement at each depot and meet their route knowledge and that diagrams are legal (e.g. driving time and PNBs). Traditionally part of the planning teams.
Resource Planners (or similar names) look 0-48 hours out to cover any gaps due to sickness, AL, vacancies based on the diagrams produced. They'll look to cross-cover roles from other depots, chop up diagrams to the staffing levels, ring round crew for overtime & RDW, or ultimately, look at what can't be covered and may have to be cancelled. Traditionally part of Control teams.