This is a tag interface (similar to Cloneable) which indicates that the contained address is "special" and is updated under the hood by the VM. The purpose is to support implementation of reflection on the current VM with these interfaces; if the Java code implementing parts of the VM requires proxies for objects in the heap, it must be the case that those proxies are updated if GC occurs. This is the level at which this updating is handled. The VM (and specifically the GC code) must have intimate knowledge of the VM-specific implementation of this interface.
Note that in the case of debugging a remote VM, it is not workable to handle the automatic updating of these handles. If the debugger allows the VM to resume running, it will have to look up once again any object references via the path they were found (i.e., the activation on the stack as the root, etc.)
