The ID of the prefab instance that contains the referenced object.
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 (GameObject.targetObjectId) and the ID of the particular prefab instance it belongs to (@@targetPrefabId@).
For more information on creating prefab instances, refer to Creating Prefabs in the Manual.
The targetPrefabId constitutes the {p} element in the string representation of a GlobalObjectId, the format of which is:GlobalObjectId_V1-{i}-{a}-{l}-{p}
Additional resources: PrefabUtility.IsPartOfAnyPrefab