This is presently a dummy template with no code. Instead, the call itself is interpreted by Module:RDIndex when it prepares summaries of the archive pages containing them. Because the code in this template page is actually empty, its presence in archives does not change how they are displayed when viewed directly. Optionally, some decorative flourish (such as bars color-coded by category) might be added here at a later time.
The tags are presently being trialled in Wikipedia:Reference desk/Archives/Lua/Science/January 2013. Categories added were astronomy, biology, chemistry, engineering, geology, and physics. Some possibly misplaced and/or possibly misclassified questions were labelled economics, network computing, pharmacy, rejected, and miscellaneous. At the moment biology and physics are the largest of these categories.
Before any large markup is undertaken there should be at least a basic agreement on the categories so that they can be used practically for sorting. For example, in this case old-fashioned science names were used - not materials science or environmental science, for example - even though these were very applicable to certain questions. It would be best to undertake the project with an agreed-upon, finite vocabulary with few enough exceptions that they can be considered individually on a talk page.
The subcategory (YYYY) is not presently listed in the archive indexes. Once a set of categories is decided on, each one can then be divided up into subcategories the same way. Hopefully, by the time category and subcategory are applied, the remaining lists of questions will be short enough that related topics can readily be spotted so that curated indexes can be prepared from all the discussions.