monetary bonus for surviving a round, which they can use to buy and/or sell body armor, ammo, and weapons at a store, also known as the Trader. The Trader is only open for a limited time between waves and in certain locations around the map. The number of waves in a match can be configured, and four difficulty levels are available: Normal, Hard, Suicidal, and Hell on Earth (ordered in increasing difficulty). A patch released in early 2016 included a dynamic difficulty option in which a computer-based "Game Controller" can alter the strength of subsequent waves, either making them easier or harder based on the players' performance. Prior to starting a match, the player selects one of several perks (or classes) that represent basic combat classes (e.g. Field Medic, Commando, Support). Each perk has various skill boosts (e.g. better damage with specific weapons, healing other characters, welding doors more effectively) that are unavailable to other perks. In the meta-game, players earn experience points for each of their perks by doing actions related to those perks, for example, healing other players will gain Field Medic experience points. The player can also acquire those same experience points when they are not using the appropriate perk. Experience points are most easily earned by using weapons specific to the class, such as explosive weapons for the Demolitions class. Each level gained boosts the perk's base skill numbers. For every five levels gained, the player is able to select one of two specific skills unlocked for the respective perk. These skills include a mix of passive and active abilities, including some that can benefit other team members. Players can also configure their playable character through a number of pre-made personas and options for various clothing and accessories, but these customizations are purely cosmetic with no impact on gameplay mechanics. In April 2016, a patch added a player-vs-player mode in which one or more players take on the role of a Zed to kill the mercenary players. The Zed abilities are implemented in game, such as a Stalker remaining invisible while not charging. Development Killing Floor 2 was developed by Tripwire Interactive, who began work following the release of their 2011 video game Red Orchestra 2: Heroes of Stalingrad. The game was announced for Microsoft Windows and Linux by PC Gamer on May 8, 2014. Tripwire president John Gibson stated Killing Floor 2 is the first