An Additional Decryption SubKey may be used for archival purposes, to encrypt for stand-ins, as well as to split subkeys to different devices. It is the remote counterpart of --encrypt-to. This feature makes use of a new OpenPGP key flag:
The "restricted encryption key" (2nd,0x04) does not take part in any automatic selection of encryption keys. It is only found on a subkey signature (type 0x18), one that refers to the key the flag applies to.