first you should understand the basic thing ,, for every customer in you company have unique number. for assign this number to customer there is two process.
1) Internal number range : system automatically assign the next available number to the customer ( so here you will not enter any number to the customer when you create)
2) External number range: you have to enter the number to the customer manually, when you create customer master.
May be you will get another question now, how to create and assign number ranges ( internal/external) to account groups..
1) create the account group with SPRO --> Financial accounts --> account receivable and account payable --> customer accounts --> master data --> preparation for creating customer master data --> define account groups with screen layout
2) create the number rage with create the number range for customer accounts (followed by above menu path)
3) if you select(tick) box in right side last column with name "EXT" , that number range will be external, if you leave the box empty it will be internal number range.
4) after creation of number range assign that number range to account type .
IF you have any further clarification ,please let me know.
Thank you so much Patchala your above explanation was explicit, A bit clarification please: What factors are considered when assigning these number range, I mean when do you choose an Internal Number Range for an Account Group and when do you choose External Number Range for an Account Group?