When someone first arrives in prison, during the reception procedure, they are classified in a security category. Adult males are placed in one of four categories, ranging from A to D. A is the highest security category and D is the lowest.

Unless classified as category A, females and young offenders do not have formal security classification, but are determined as suitable for either open or closed conditions.
Someone’s security category is determined by their risk to the public should they escape and their likelihood of escape, rather than by the nature of their offence.
Different prisons normally hold (although not always) prisoners of a specific security category.

The different categories

Category A Prisoners classified as Category A are those for whom escape must be avoided at all costs as they pose extreme danger to the public. They are placed in top security prisons.

Category B Prisoners classified as Category B are those for whom the maximal conditions of security are not necessary but for who escape must still be made very difficult.

Category C Prisoners classified as Category C cannot be trusted in open prison conditions, but are seen as not having the resources and motivation to make a determined escape attempt.

Category D Prisoners classified as Category D are those who can be reasonably trusted in open conditions.
Prisoners serving longer-term sentences should have their security category reviewed at regular intervals. By the time a prisoner is released they should have moved down to category D.