A retainer is normally an amount paid by a tenant to a landlord or their letting agent to retain a property prior to formal agreement to rent..
Summer rent if reduced is a retainer during un-occupancy.
The tenant is not in residence but the property is nevertheless retained for the tenant for a price.
Simply because some rent is paid does not necessarily entitle a tenant to occupy during the reduced rent period.
The landlord is entitled to carry out maintenance during retainer periods. This benefits both landlord and tenant enabling unrestricted maintenance access for the landlord and tradespeople, minimising disruption following the tenant taking up residence.
If full rent is charged during the summer, the tenants are normally entitled to occupy from the first date of the tenancy.
If any reduction is offered then the tenant is not necessarily entitled to occupy and the extent of any occupancy is determined by the terms of the contract (AST).
The tenancy agreement may not specifically mention the word retainer. It may simply be implicit by stipulating any rent reduction during specific periods say during the summer from July to August.