An End of Tenancy Clean is when a tenant leaves a property; they should theoretically leave it in tip-top condition ready for the next new occupier, although in reality they often don’t. Even if they do to some degree, any new occupier will be expecting it to both look and smell freshly cleaned.
Therefore tenants or landlords or managing agents can instruct these one-off intense cleans of the whole property, often on a tight time scale between lettings, and often needing reports on what was involved and any issues such broken items spotted in order for the parties to agree who is responsible for these through the letting agreement.