HMRC writes the rules for tax collection; some rules are to raise money, some to promote certain types of behaviour, some to kerb behaviour and some are just political spin.
It is our job as accountants to apply/comply with these rules, but also to play these rules to your best advantage and, HMRC hopes, in the way they planned.
Certain parties specialise in finding the holes in these rules and exploiting them. The systems they devise to exploit the loopholes are known as tax avoidance schemes.
HMRC knows that these schemes exist and so has set up a system whereby a legal scheme is registered with HMRC. If you use such a scheme you have to say so and quote its registration number. In this way HMRC can track which schemes are costing them too much tax.
HMRC then has a choice. They can challenge the scheme, taking the scheme developers to court to try and prove it does not work, or they can change the law to close the loophole. Sometimes they do both.
If a scheme is taken to court then all the registered users of the scheme get a tax enquiry, until the result of the court case is known.
The new development is that now HMRC can demand that the tax (the tax that is saved by use of the scheme) is actually paid to HMRC pending the result of the court case.