When you create a customer in VD01, you need to specify one account group. In the example below it is DEBI.
When this customer is saved, it is stored in table KNA1, field KTOKD.

In table CRMKTOKD, the mapping relationship between account group in ERP and CRM is maintained. In this example, the corresponding group in CRM is 0002.
( See function module PI_BP_GET_CLASSIF_FROM_KTOKD )

When the customer is downloaded into ERP from CRM, the business partner will be created with a proper number. The number range is maintained based on mapped CRM partner group, in this example 0002:

Here you assign the number range setting to the mapped group 0002:

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Account group in ERP and its mapping relationship with CRM partner group