Master-detail child reparenting
Key Takeaway
By default, a detail record can't move to a different master record. An app builder can permit the move by selecting "Allow reparenting", and the other master-detail rules still apply.
In a master-detail relationship, each detail record (the child) is tied to a master record (the parent) that controls its deletion, ownership and access. The Allow reparenting option decides whether a detail record can move to a different master record.
What reparenting means
Reparenting means changing a detail record in a master-detail relationship so that it points to a different master record. By default, you can't reparent records in a master-detail relationship.
For example, imagine a custom Inspection object on the detail side of a master-detail relationship, with a custom Building object as the master. By default, you can't change an Inspection record so that it points to a different Building record. After an app builder selects the Allow reparenting option, you can make that change.