In practice several terms appear around the conclusion of a contract that have different legal consequences. The purchase offer is the binding offer of one party. If it is accepted, the purchase contract is concluded. The label as offer, purchase offer or purchase proposal changes nothing about the binding effect.
The written record of the main points is a written fixing of the main points of a transaction, that is above all the object and the price. If the parties agree on these main points and fix them in writing, the contract has already validly come into being, even if secondary points are still open. A signature meant as such a record can therefore already bind.
The preliminary contract is the obligation to conclude a main contract in the future. From it one party can demand performance, that is the conclusion of the main contract. The preliminary contract too therefore binds, even if in a different form than the immediately effective offer. Which type of contract is present in the specific case follows from the content of the declaration, not from its heading.