What is it?
A challenge cache is a cache that requires the player to fulfill a geocaching goal before they are allowed to log the cache as found. Challenge caches have to be mystery caches and have the challenge attribute, they also must have the word Challenge in the cache name. Per the guidelines every challenge cache published after April 21, 2015 have to have a challenge checker.
More information about challenge caches can be found in the challenge cache guidelines.
Challenge caches at Project-GC
By default, all geocaches with the Challenge-attribute will be considered as challenge caches. However, an attribute could be placed on a non-challenge cache by accident or with malicious intent to "improve" the statistics. A cache owner could also forget to add said attribute to a challenge cache. The reviewer should review for incorrect use of the challenge attribute but they might miss this during review or the cache owner might add or remove the attribute after the cache is published.
To solve this problem Project-GC uses a list of Challenge exceptions to make sure that the statistics regarding Challenge caches is as correct as possible. Project-GC also has a system for disabling the challenge checker if the cache lacks the Challenge-attribute.
The Challenge Exceptions-system is a list of caches that need to be checked manually by volunteers from the Project-GC-community to see if they are challenge caches or not. The process is simple, the volunteer checks the cache description to see if the cache is a challenge or not and then uses an 'up-vote' to say that it is a challenge or a 'down-vote' to say that it is not a challenge. The vote should be placed based on if the Geocache requires the finder to do something before/when logging it and not if it is a 'good' challenge or if it complies with the current guidelines.
Only geocaches with a confidence score of at least 35% will be be considered as an exception, the more homogeneous votes, the higher the confidence. For example 3 homogeneous votes should give a high enough confidence score, while for example 3 up-votes and 1 down-vote will not be enough. There is a forum to discuss special cases and believes related to these exceptions. The forum is readable by everyone, but only those with voting access can write in it.
Caches are added to the exceptions list automatically via a set of criteria but can also be added manually, the easiest way to suggest exceptions is via the dedicated thread in Project-GC's Community support system.
Challenge checkers are the main Project-GC tool related to challenge caches.