Until very recently there was a slightly complicated scheme whereby certain nationalities on certain types of visa had to register with the police when they arrived in the UK, and they would obtain a document called a Police Registration Certificate (PRC). This document had to be updated if for example they changed their address or changed or extended their visa.
We were never really sure what purpose this scheme served and evidently the Home Office is not sure either, because they have just abolished it. The situation is rather confusing because, at the time of writing, the scheme still appears in the Immigration Rules.
But do not worry: the scheme is dead and there is no longer need to follow it, and it is no longer necessary to submit your PRC in support of an in-country visa application.
This has made life slightly less complicated for a lot of people.
Level 3 Immigration Adviser