A little background. This was added by the old PGP Inc company for commercial users so as to escrow email. If a key had this subpacket, you would encrypt to that additional key as well.
The notion was that it should go in the standard, but that was politically charged at the time - indeed Loius Freeh stood up in front of Congress and used this very feature as proof that it was possible to force all crypto programs to escrow messages for the FBI...
The compromise that was reached was that it not be documented in the standard. I don't know if GPG implements it, or even if it the PGP line still includes it. I think architecturally speaking, such a feature is better off in the proxy products, and layered over the top at the admin level rather than put in the tech. I think it is relatively safe to ignore it.