I'm working on an architecture document based on quality attributes. I'm trying to explain our search algorithm based on tags, historics and some bi information as a way of favouring a quality attribute. I read some material about this (Software Architecture in Practice, Third Edition) : usability is the closest I got (in the way of making it easy to search for something) but that is not quite right. I've read a list of quality attributes in Wikipedia:  maybe correctness or relevance would apply? are those even formal quality attributes?
What quality attribute would be favored using a rich search algorithm?
architecture;documentation;enterprise architecture;quality attributes
If you're looking for a formal attribute, you'd better stick to ISO/IEC 9126 or its successor ISO/IEC 25010.  Following are functionality attributes:  correctness corresponds to accuracy. This means that it produces the correct results and with sufficient degree of precision.  relevance seams to match suitability.  This means that it provides the right functions for the user's task and objectives.  I understand you want to highlight how your algorithm is favoring/contributing to quality: not sure that correctness is a good match here, unless your algorithm has some feature that must be used to achieve correct results.  relevance could be a good match as the algorithm will somehow make the searching more efficient, helping user to better achieve his/her goals. Usability is a group of several attributes, that are about ease of use and attractivity. It's too general for a document about quality attributes.  The following would be more precise, and seem also to suit your needs:  operablity is about facilitating the user to control the software while meeting user's expectations (e.g. better, more targeted search results ? thus avoiding lots of browsing across correct but less relevant results)    attractiveness is about the emotions of the user.  For example, if this algorithm will make the user like your software more than comparable software using another search algorithm.  