The coexistence master data management approach may see data from the transactional masters across several systems, being consolidated to a single customer view of the customer master.
This is typically updated posthoc and periodically synchronized reflecting all the external keys.
The content of the master from various systems may be inconsistent and more importantly, no single entry is necessarily considered to be more correct and appropriate than any other.
Conflicts between records can and do occur and because the authoring is undertaken in different systems, the end result may only be useful for generalized reference and may not be considered a true or absolute authority.