PPtr

This page needs more research, and is partially based on speculation.

This is a type that is used to reference Unity Objects in the Unity serialized data format1). It is likely that this is a type only on the native side.

This type can appear in error messages as well as when querying a SerializedProperty.type. 2)

1)
ie. Scene Files, Prefabs, etc
2)
which may return strings such as PPtr<EditorExtension>