By this we mean the legal document relating to your proposed sale or purchase. These will set out the various terms and conditions with the exact wording agreed between the two legal advisers as appropriate. Once these terms are agreed, both seller and purchaser sign their copy of the contract. When these two, then identical, documents are exchanged, the deal is settled and becomes binding on both parties. You cannot change you mind!
Until then, while the transaction is still ‘subject to contract’, other negotiations could continue – indeed we are obliged by law to notify a seller-client of any other offer made for their property right up to the final exchange of contracts. Remember this is a private treaty negotiation and details of the various offers are not disclosed to anyone else interested in the property, technically without agreement – if all parties agreed that is a different matter. Usually they do not.