It really could not get more simple than this, but all too many people insist on confusing themselves on this point by listening to bad advice peddled by people who supposedly have closed this deal or that deal or 50 deals on their word.
The LOI as it is now used is a misunderstood application of poorly informed brokers and traders. A more proper application based on established custom and practice in International trade, going back even to the old medieval "Law Merchant" (Lex Mercantilis) isɅ. Buyer requests a quotation _ in modern usage this is an RFQ/Request For Quotation _ NOT an LOIǎ. Seller may then reply with a Quote. Buyer confirms receipt of quoted prices, and requests an OfferǏ. Seller replies with an offer (the firm or full offer is the so_called "FCO" in Internet Broker_speak).