When custom fields are migrated from server to cloud, the IDs change. If the IDs are not updated in other entities that reference the custom field, the entity will not function properly and/or even result in a migration failure. eg. a screen scheme that specified a custom field in to be displayed in Jira Server, is not updated with the new custom field ID when it has been migrated to Cloud.
Customers want and expect the migration to maintain referential integrity for all migrated custom fields, including app custom fields. This ticket is about sending the mappings as objects IDs change as they move from Server to Cloud, so that the references can be updated and functionality preserved as the overall outcome of moving the selected project data from Server to Cloud.