What is a MsgPack 'zone'

msgpack_zone is an internal structure used for memory management & lifecycle at unpacking time. I would say you will never have to interact with it if you use the standard, high-level interface for unpacking or the alternative streaming version.

To my knowledge, there is no detailed documentation: instead you should refer to the test suite that provides convenient code samples to achieve the common tasks, e.g. see pack_unpack_c.cc and streaming_c.cc.