It is generally recommended practice for council sites to follow some content classification schemas to help drive various elements of the site.  The main ones to be concerned about are called IPSV (Integrated Public Service Vocabulary), & LGNL (Local Government Navigation List).

There are more, a lot more!  But these two are the ones we most commonly use on council sites - so they are the ones I understand! 

I'll try my best to explain what they are and do, and why they are important.  There is a full list of all of them with the relevant files to upload schemas (XML) at: http://www.esd.org.uk/standards/

So... 

IPSV: the basic idea is that all pages should be tagged.  Much as people tag blogs it can help with the general navigation and finding information on the site.  With Local Government sites it is a good idea to have all of the sites tagged with terms that are shared across all other Local Government sites (and with IPSV shared with all Government services - so fire, police etc).  This allows specialist government search engines to pick the correct information for the locality that you are in, and to allow sites to deep link into the correct information within your local government site. 

IPSV is the tagging schema for government services.  It is a hierarchical structure that can be seen here: http://www.esd.org.uk/standards/ipsv/viewer/viewer.aspx

(note that their site wouldn't expand the levels for me in Chrome (v2) but works fine in IE8).  The interesting stuff comes around the idea of related terms.  So for example if you pick Home Ownership as the term for a page, then there is a list of related terms which can drive either other tags being added automatically (with the content editors approval) or drive other content on the site (so you are looking at the Home Ownership page and you get a related links panel that shows Stamp Duty for example)

When we wrote a system to do this, we also had some complex rules around preferred and non-preferred terms being automatically added, and the idea of parent tags being used as well.  (I can try and expand if needed but I'm a little rusty). 

LGNL is an attempt to standardise the navigation on all Local Government sites.  It basically defines what users should see in the content tree, and how that should be structured.  It means that if you go from one council site to another you will be able to follow the same path and get the local information for that topic in the same way.  It also removes some of the major inconsistancies with content editors placing pages where they "think" they should go.  

However, people do think about where an item of content should live in different ways, so LGNL has been designed as poly-hierarchical, i.e. each term can exist in many places.  LGNL can be driven from IPSV (above) and there is a mapping file between the various terms.  It gets relatively complex as each IPSV term may have multiple LGNL terms which may exist in multiple places in the content tree.  Which means coding the navigation structure is quite tricky!