A deposit is usually a sum of money received before a supply takes place. It is an advance payment for the supply and, as such, creates an actual tax point on the date of receipt.
For example, a VAT-registered builder requires a customer to pay a £1,000 deposit before he starts repairing a roof in order to fund the purchase of materials for the job. The balance of the price is due when the job has been completed.
The builder must account for output tax on the deposit on the date he receives it because the intention is that the deposit is a part-payment towards the building services (the same principle applies to goods)