Business Unit structure is a basic of the security structure on Dataverse. In most of the case it depicts the organisation structure of the business. Standard behaviour of Dataverse is that one user can be part of only one business unit. There are business constraints around that for long time were a user cannot be part of multiple business unit. This can be mitigated until now using the teams access.
The new modernized business unit structure solves the problem of user part of multiple business unit using the security role instead of the teams. Now we can able to assign security role of different business unit. In the previous iterations, the security role business unit are auto determined by the business unit of the user which he/she part of. The new approach will help to choose the security role and the business unit that we need to allot.
The new modernized business unit also cater to select the record owning business unit which were read-only fields earlier. Those are populated based on the user/team which creates that record. The cascading effect for the child records are also controlled for user and business unit changes.
https://powerapps.microsoft.com/en-us/blog/announcing-public-preview-for-modernize-business-units/
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