An IPS tag was implemented by the UK registry (Nominet) to control domain name transfers. Every UK domain name (.co.uk, .org.uk, .net.uk, etc.) must have a IPS tag specified as to confirm which registrar currently holds the domain. This protects domains from being transferred away from a current owner if the domain's tag has not been updated to another registrar's tag. Only the current tagholder (registrar) or Nominet is able to update a domain's IPS tag.
To explain further, every UK domain registered with 1&1 will use the 1&1 tag (1AND1). As long as this domain's tag remains 1AND1, all domain transfer requests will be ignored as the tag specifies the domain should remain with 1&1. If the owner of this domain would wish to transfer the domain to another registrar, they would have to update the IPS tag through 1&1 to specify the tag of the gaining registrar.
Once the tag has been changed with the domain's current registrar, the domain will then be permitted for transfer to only the specified tagholder (registrar).