There are a number of rules, regulations and guidelines by which prisons are run. These are outlined in Prison Service Instructions (PSIs) and Prison Service Orders (PSOs).
Prison Service Orders (PSO)
These were issued until 31 July 2009. They were mandatory instructions with no expiry date, and remained in force until cancelled. Existing PSOs will remain in force until replaced.
Prison Service Instructions (PSI)
From 1 August 2009, all Prison Service operating instructions are published as PSIs. They have a fixed expiry date. Prior to 1 August 2009, PSIs were issued to convey short term instructions or amendments to PSOs and had a maximum validity of 12 months.