Attributes are small icons that are set by the cache owner to help the player prepare for the finding of the geocache. They can indicate for example if there is tree climbing required to find the cache, if the cache is not recommended for tourists or if the player should watch out for livestock. There are 70 different attributes available, but 41 of them also have "negative state" (i.e. being crossed out to mean the opposite) making a total of 111 possible attributes. More information about attributes can be found in Geocaching.com's Help Center.
Attributes and Project-GC
Project-GC uses the attribute "Challenge cache" as the main way to determine if a cache is a challenge cache or not, but it also uses a list of exceptions to compensate for the abuse of the attribute.