to upload user-created content, such as maps, weapon skins, and custom game-modes. Some popular user-created skins are added to the game and are obtainable from unboxing them in cases. The creators of the skins are paid when their item is added to a case. These skins helped form a virtual economy in Global Offensive, leading to the creation of gambling, betting, and trading sites. The addition of skins and the associated virtual economy launched Global Offensive's player count past the other games in the Counter-Strike series and is one of the most important updates in the game's history. Events called "Operations" are held occasionally and can be accessed through purchasable expansion packs in the form of "operation passes." These passes grant access to operation objectives which are spread over different game modes, such as Arms Race and Deathmatch, or in operation-specific game modes, first seen in Operation Hydra, released in May 2017. Completing these challenges rewards the player with XP and the ability to upgrade the operation "coin." The maps in the operations are community made, meaning some of the revenue made goes towards the map designers. An update in October 2014 added "music kits", which replace the default in-game music with music from soundtrack artists commissioned by Valve. If a player with a music kit equipped becomes the round's most valuable player, their music will play for others at the end of the round. There is a feature that allows kits to be borrowed, and kits can be sold and exchanged through Steam's Community Market. In 2016, the game saw two remakes of original Counter-Strike maps, as well as the introduction of Prime matchmaking and additional items. As a part of the Operation Wildfire promotion, Nuke was remade and re-released in February with the primary goals being to balance the map and make it more aesthetically pleasing. In April, Prime matchmaking was added to the game. To partake in this mode, the user had to have a verified phone number connected to their account. It was introduced in an attempt to prevent legitimate players from playing with cheaters or high-skilled players playing on alternative, lower ranked accounts, a practice colloquially known as "smurfing". Inferno, another original map, was re-released in October. Valve said they had three reasons behind the remake: "to improve visibility; to make it easier to move around in groups; and to tune it with player feedback."