The supervised discharge provisions are abolished by the Mental Health Act 2007. See Supervised Community Treatment replaces Supervised Discharge 3/11/08 for details of this, including the transitional provisions.
Patients who had been detained under s3 (and unrestricted hospital orders) could be given a supervised discharge under s25A. This was also called being made "subject to aftercare under supervision". Its aim was to help break the cycle of repeated admissions, relapses, and readmissions of "revolving door patients".
The supervised discharge took effect when the application had been (a) made by the RMO, (b) accepted by the Health Authority, and (c) the patient has "left hospital": s25A(2). For a patient on s17 leave, "left hospital" refers to when the leave expires: s25A(9). In theory, the s3 should be formally discharged (e.g. under s23).
Just like s3, the s25A initially lasted for 6 months, could be renewed for a further 6-month period and then annually thereafter. NB. For the purposes of calculating eligibility periods and renewal dates, the s25A actually begins as soon as step (b) above has been completed. A s3 patient made (or about to be made) subject to s25A may, if the timing and circumstances were right, be able to keep his hearing date by making a separate appeal against the s25A. Cf R (SR) v MHRT  EWHC 2923 (Admin).