This is the shortform name by which Nominet recognise a provider of .uk domains - you should get this from them or their help information on transferring domain name to them.
Hosting companies which host .uk domains are required to have a Nominet Provider tag.
Unless you are wanting a .uk domain name then, AFAICS, you do not need this information.
I found some very helpful information after a long search on Magix.com
In order to transfer your domain, you will need to cancel it with us. This will then provide
you a transfer authorisation code that you can give to the new provider.