I think this is referring to the ability to the tagged literals api (set data-readers[1] and change/extend the various literal notations already provided by clojure (like #uuid and #inst)). The docs seem to believe that unqualified var names are reserved and you should fully-qualify your tags, but this seems to be more of a suggestion than a requirement.

[1][2]

    *data-readers* 

[2] http://clojure.github.io/clojure/clojure.core-api.html#cloju...