Your implementor is correct, the customer/vendor ID is generated centrally. The questions is why do you need it based upon division? There are other flags that segregate this. You would anyway need a separate sequence because you could not have the same number twice (even if split by division).
We have used combination of continous and manual. Before the loading of masterfiles(templates) both for customer & vendor, we have set the number sequence to MANUAL then we have assigned each vendor & customer respective codes. After uploading the master lists we set the number sequence to CONTINOUS. Please do not forget to adjust the number sequence particularly the “NEXT” field for the next number in the series after loading your masterfiles.