A reservation document is an option to purchase contract in which the buyers commit themselves to acquire a particular property and the vendor to sell that particular property.  In this contract the general conditions of the purchase are drafted and it can be used for the purchase of a resale property or a new build home.  The primary function of the reservation contract is to set the basis of a property purchase and take the property off the market, so that no other person can purchase the same unit.
Any reservation contract will last a determined amount of time, in Andalucía, it is usually for a period of two or three weeks or until both parties involved sign a Contrato de Arras Penitenciales (Private purchase contract).  In certain occasions both buyers and vendors agree to go from signing of the reservation contract directly to singing of the title deeds, in these particular cases, the reservation document lasts until completion at notary.
On the Costa del Sol, it is standard that the reservation is accompanied with earnest money and the amount is normally of €6.000, which is normally held by the estate agent or the buyers´ solicitor and is not transferred to the vendor or their solicitor until the due diligence is carried.  Once all the necessary verifications have taken place by the buyers or their solicitor, the buyers will allow transferring this deposit to the vendors or their solicitor´s escrow account, the next step of the purchase is normally to sign a private purchase contract.
Pro Tip:  On the Costa del Sol for the reservation contract to be valid it has to be accompanied with a deposit payment.  If the deposit has been paid and the buyers decide not to continue with the purchase, the deposit will become non-refundable.  It will only be refundable in the event of a legal problem with the property that makes the transaction impossible.  On the other hand, if the vendor has received the funds and decides not to continue with the purchase, they would have to pay double the amount received to the buyers.