Classic & EC - PO & other purchasing documents number range

Hi SRM Gurus,

Can any one please explain me more precisely about number ranges in SRM & MM.

For classical scenario, we will define number ranges for PO & other purchasing documents in SRM as internal number ranges and in MM as external number ranges.

But in Classical scenario we will create POs (or other Purchasing Documents) in backend system. Suppose If it is to be replicated to SRM as per business need, then how system works. Because we defined in SRM PO number range as internal number range and in backend system as external number range.

Again in Extended Classical scenario, for MRP materials PR or Reservation will be created in backend system with internal number range. But in Plan-Driven scenario they will be replicate to SRM. In this case how we have to choose number ranges.

I am confused. I'll appreciate if any one explains me with step by step or with examples for Classic and Extended classical scenarios

Points will rewarded

Thanks,