The National Standards for Youth Justice Services has guidelines for Section 90/91 from page 129.

Section 90

If the conviction is for murder, the sentence falls under  Section 90 of the Powers of the Criminal Courts (Sentencing) Act 2000 Section 90 of the Powers of the Criminal Courts (Sentencing) Act 2000. Such sentences are termed "Detention at Her Majesty's Pleasure" and a mandatory life sentence will apply. The sentencing court will set a minimum term (also known as the tariff) to be spent in custody, after which the young person can apply to the Parole Board for release. The  Secretary of State’s directions to the Parole Board (issued August 2004) - Secretary of State's directions to the Parole Board (issued August 2004) set out the assessment criteria for the release of those serving a life sentence. Once released, the young person will be subject to a supervisory licence for an indefinite period.

Section 91

If a young person is convicted of an offence for which an adult could receive at least 14 years in custody, they may be sentenced under Section 91 of the Powers of the Criminal Courts (Sentencing) Act 2000 Section 91 of the Powers of the Criminal Courts (Sentencing) Act 2000. The length of the sentence can be anywhere up to the adult maximum for the same offence, which for certain offences may be life.

A young person given a Section 91 sentence will be placed in custody. The young person will be released automatically at the halfway point and could be released up to a maximum of 135 days early on a  Home Detention Curfew (HDC) Home Detention Curfew (HDC), if they meet the eligibility criteria for the scheme and pass a risk assessment (authorised by the Governor for those in a YOI or the YJB Placement and Casework Service for those in STCs or secure children's homes). Once released, the young person will be subject to:
a supervisory licence until their sentence expires, if the sentence is 12 months or more
a Notice of Supervision for a minimum of three months, if their sentence if less than 12 months.