The function is exclusive to Playstation.
Alex Hutchinson, Creative Director of Far Cry 4, wanted to explain how the fact of being able to "share" with your friends the co-op experience of the title announced during E3 2014 works and he does it with these words:
“When a player buys the version Playstation 4 o Playstation 3 di Far Cry 4, will have approx 10 "ticket" named "Keys to Kyrat", which can be given to friends. They will be able to download a limited version of FC4 which has all the co-op content but not the single player content. They will be able to play the limited version if the passer-by is playing the main version of FC4. " Hutchinson isn't sure, but Sony could implement some sort of time limit.
Far Cry 4 is outgoing on November 18, 2014 su Playstation 3, Playstation 4, Xbox 360, Xbox One and PC.
Update: time limit actually implemented, you can play with the ticket for a maximum of two hours.
