An assignment is the transfer of a tenancy by the tenant to another person. The Housing Act 1985 sets out when this can be done. Unless an assignment is by court order or under the right to exchange, tenants can only assign their tenancy to someone who has a legal right to succeed and to be valid an assignment must be done by Deed. For these reasons this guidance needs to be prescriptive.
You can read the full guidance here.
As well as specifying when an assignment can happen the HA 1985 says that tenants must get their landlord's permission. Landlords, however, can't unreasonably refuse a request. Our tenancy conditions give examples of what we take into account when considering a request.