From 1 April 2011 company tax returns and accounts for accounting periods ending after 31 March 2010 must be filed online using the iXBRL format. HMRC have updated their tagging guide for iXBRL, which explains when to tag, how to tag and what to tag.
However, if commercial software is used, the tags are inserted 'behind the scenes', meaning that a details knowledge of the tags is not required.
The tagging guide is available on the HMRC website.