public ulong targetPrefabId { get; }

Adding a new prefab instance to a scene creates a new instance of every object the prefab contains, including every GameObject. The GameObject inside a prefab instance doesn't have its own stable local file ID and a single prefab can be instantiated multiple times in a scene. Therefore, to reliably identify a GameObject that's part of a prefab requires both the local file ID of the original version of the GameObject (targetObjectId) and the ID of the particular prefab instance it belongs to ().

For more information on creating prefab instances, refer to Creating Prefabs in the Manual.

The constitutes the element in the string representation of a , the format of which is:

GlobalObjectId_V1-{i}-{a}-{l}-{p}

Additional Resources: PrefabUtility.IsPartOfAnyPrefab