User-create labels that are not controlled as filters or filter groups.
In Information Architecture, a "folksonomy" is a set of user-created terms that do not come from an official controlled taxonomy (the term is a portmanteau of "folk" and "taxonomy"). These terms can then be used to tag and label content. Hashtags on Twitter are a good example of terms from a folksonomy.
Folksonomy labels can complement official terms in important ways, especially if the official terms are lacking in specificity or currency. On the other hand, user-entered terms are prone to typos, misspellings, concept duplication, intentional deception, and other issues. In short, folksonomies should be both encouraged and managed.
In Brikit's Targeted Search version 2.0+, labels that do not come from the pre-defined filter groups are considered part of the folksonomy. Folksonomy labels can be "promoted" to become official filter terms, or replaced with an official term, or ignored altogether.