FDICT (Preset dictionary)
         If FDICT is set, a DICT dictionary identifier is present
         immediately after the FLG byte. The dictionary is a sequence of
         bytes which are initially fed to the compressor without
         producing any compressed output. DICT is the Adler-32 checksum
         of this sequence of bytes (see the definition of ADLER32
         below).  The decompressor can use this identifier to determine
         which dictionary has been used by the compressor.

Well, wow. I have to wonder why this wasn't more utilized, then. There are a ton of contexts (columnar data in databases, for example) where shared-dictionary-based compression might have helped a ton before now.