Every object in the Editor has a distinct GlobalObjectId. This is (effectively) the identifier Unity uses to hold references to other objects. It is guaranteed to be unique and stable between editor sessions.

GlobalObjectIds are not available at runtime. However, they are available in Play Mode in the editor. If you use these in play mode, keep in mind the notes on prefabs below.

All notes below only apply to running the game in Play mode in the Editor.

Prefabs Instances have a GID value that is different from the GID assigned when the game is run in Play Mode. When a scene is loaded in play mode, prefabs are merged (unpacked) into standard game-objects. These gameobjects have a different GID than the original prefab. However the GID of the unpacked GameObjects is *still* unique and stable!

Whatever process that Unity uses to unpack gameobjects¹⁾ must be determinstic, and generate a fileID for the new gameobject that is based on the prefab.