These are used when you want to evict a tenant either at the end of the fixed term or when using a break clause to regain possession of your property, to serve a Section 21 Form 6A notice you do not to give grounds for ending the tenancy agreement.
If your property is being rented under an Assured Shorthold Tenancy and you are a landlord trying to regain possession of your property it may be necessary to serve a Section 21 Form 6A notice. Under the Deregulation Act 2015 a Section 21 Form 6A notice only has a lifespan of 6 months and can not be served before 4 months of the tenancy has elapsed.