Explanation
Graphical user interface, text Description automatically generated

Box 1: Existing accounts show the new BPF.
When an entity record is being created and if there are multiple BPFs defined on that entity. The system would do the following:
If the ProcessId field is set to Guid.Empty. The system will skip defaulting the BPF on that instance.
If the ProcessId field is set to specific BPF entity reference. The system will default to the specified BPF.
If the ProcessId field on the record is not set. The system will default the BPF.
Box 2: No BPF is linked to a new account.
Note: A business process flow definition is represented as a custom entity and an instance of a process is stored as a record within that entity. Each record is associated with a data record (such as an Account, Contact, Lead, or Opportunity) and in case of cross-entity processes, with a data record for each participating entity.
Reference:
https://docs.microsoft.com/en-us/dynamics365/customerengagement/on-premises/customize/business-process-flo