TElPGPPreferredKeyServerSignatureSubpacket is a descendant of TElPGPSignatureSubpacket class.
The following paragraph is taken from RFC 2440 - OpenPGP messages format, part 220.127.116.11.:
«This is a URL of a key server that the key holder prefers be used for updates. Note that keys with multiple user ids can have a preferred key server for each user id. Note also that since this is a URL, the key server can actually be a copy of the key retrieved by ftp, http, finger, etc.»