what does posix/Europe/Paris stand for?
Generally, you shouldn't be using the Posix/ or Etc/ timezones if they're on your system. This isn't a PostgreSQL thing, it's your distributions libc database (sometimes packaged as tzdata/zoneinfo) for the internal timezone functions. Most of that POSIX stuff is nasty and old. PostgreSQL has a document on it which mentions it.

I guess to answer the question specifically, 


  Two different versions are provided: - The "posix" version is based on the Coordinated Universal Time (UTC). - The "right" version is based on the International Atomic Time (TAI), and it includes the leap seconds.


There really is no reason that I know of not to use the IANA names.