[2] The response indicates how government departments and agencies and their staffs should react to each threat level.

[5] The Joint Terrorism Analysis Centre (JTAC) is responsible for setting the threat level from international terrorism and the Security Service (MI5) is responsible for setting both threat levels related to Northern Ireland.

[1] The threat level informs decisions on protective security measures taken by public bodies, the police and the transport sector.

Following terrorist attacks in Indonesia in 2002, the system was criticised by the Intelligence and Security Committee of Parliament (ISC) as insufficiently clear and needing to be of greater use to "customer departments".

The following table records changes to the threat levels from August 2006 – July 2019 before the 'New Format' was put into place:[1] Historic/Defunct: