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.
To be valid an assignment has to be by Deed. This is a formal legally binding document. As the Deed must follow a prescribed format please read the full assignment guidance note if dealing with a request.
A range of guidance notes on what to update on NEC can be accessed on this link