Whitelisting are approved extensions to the gameplay, they well allow perk progression and allow achievements to be unlocked. You needed to submit your mod for approval for this.

Greylist allow players to use their perks but will not gain/level them up or unlock achievements.

Blacklist does not allow players to use their perks at all (you will be level 0) or unlock achievements. This was the default. This tier is usually given to those that did not obtain whitelisting statues.

When the Hillbilly horror update (2012) went live, TWI removed Blacklisting on mutators, defaulting any mutator to grey.

Maps also had to be listed but was handled by other devs.

• Maps no longer have whitelisting?
  No, TWI completely removed listings of maps, defaulting any map to whitelisted.
  
  
• Well Mutators ever remove the whitelisting?
  Never, mutators well always have this security. Its obvious why
  
  
• So what mutators are whitelisted?
  You can see all the current whitelisted files here:
  http://kf-wiki.com/wiki/List_of_whitelisted_mutators
  or in my collection:
  https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=117038038
  
  
• How can a mutator be whitelisted?
  At Tripwire Interactive's forums, a developer would post a submission thread where the mod creator will post either their map or mutator/scripts where they will be reviewed for approval. The developers however would generally not announced which submitted item has been approved, leaving the players to test every single submitted mod.
  
  
• When is the next Whitelist roundup (Submission)?
  NEVER, this game has been declared finished. If the game is no longer being updated, then there is no way in updating the whitelisting.

Clientside mods, as the name says, are mods that are used by the client (you) and only used by YOU! You can go online and play with these and nothing well go wrong...hopefully.

• So are all character skins clientside?
  If they replace the a upl file then yes, its just being told what textures to read.
  
  
• What is a Mismatch?
  Mismatch is caused when the client/server is using a different version of a file and because of this, prevents you from joining a server. Every UE file has its own unique serial code. These are always checked.
  
  
• How can i make a file prevent mismatch and be Clientside?
  This is something i can not say, but im sure you have seen mods like these (The Merchant Trader/ Zoey Trader/ No Film Grain/Reskin Zeds/etc). The way these are set up are a 'secret'. Textures, sound, animations, static meshes, scripts are all under this strict check.
  
  
• So are their any simple things i can make clientside?
  Yes, you can make a custom texture package for loading screens since these textures are only read through the user.ini. Music is also exclusively used under the client. Music is never checked by the server which means the player can make any song at any length so long as its a filename a map actually reads. INT files which are just text files can also be altered as they are also only used by the client.