This term refers to any structure including a company that is based offshore (i.e. not in the UK) that offers services to UK recruitment agencies and/or freelancers.
Generally, companies that are incorporated offshore fall outside of the remit of HMRC, meaning, in some cases, even though they are engaging workers in the UK, the offshore entity is able to avoid paying the correct level of UK tax and National Insurance, resulting in a loss in tax revenue to the Exchequer.
Not all offshore companies use their location to avoid paying tax, but those that do place their workers, and the entire supply chain at risk of debt transfer under the Offshore Intermediaries Legislation.