Clojure: Meaning of ::tag

Clojure: Meaning of ::tag

Recently I met construction like {::tag 10} in Clojure. What does it mean?
My experiments showed up that it's a keyword to:
=> (type :tag)
clojure.lang.Keyword
=> (type ::tag)
clojure.lang.Keyword
The difference is value itself:
=> :tag
:tag
=> ::tag
:/user/tag
Seems like ::tag is namespace qualified. Is it right guess? If yes, what
the difference between namespace qualified keyword and non-qualified? When
it can be useful?