The concept of customer master which is designed by SAP requires a mapping between the BP Grouping in CRM to the Account Group in ECC.
1. As for most of the business processes (client implementations), ECC is the Master System for Customer creation and maintenance.
2. For the seamless flow of these customers to CRM, the mapping of Account Group and BP Grouping needs to be done in ECC through T-Code - PIDE (in ECC)
3. Every customer which is created in ECC should get replicated to CRM and it should have the same number. Hence the number range is maintained against an Account Group in ECC and the same number range is been mapped to CRM Grouping in CRM.
In CRM, the mapping between Number Range and Grouping is done in following path
SPRO > Cross -Application Components > SAP Business Partner > Business Partner > Basic Settings > Number Ranges and Groupings
Hope this will help
You can also refer to the below help links, for more information on Number ranges and PIDE settings:
Hope this helps.