If you have custom manager layout and want to perform some validations before data is saved to the DB, you can use this feature.

Example scenario: Develop a custom manager layout for entering PO details where user can either select pre-existing location as destination location or can enter complete address details like address line1, zip code etc using custom attributes.

In this custom screen, if we want to make sure that user either enters Destination Location XID or Enter new Destination location details with address details for the destination address.