An offer is half a trade
Write your half and sign it — that costs nothing and touches no chain. Send it to whoever is on the other side. Only their fill is a transaction, and it moves both sides or neither.
Who can take it
Partial fills
You will be asked to approve exactly what you are giving, then to sign the offer. Signing is free.
Foil reads the offer out of the link itself, then asks the chain what is true about it now: whether it has been cancelled, whether it is expired, whether the maker still holds what they promised.
Offers you have written here
Kept in this browser only — Foil has no server and no idea who you are. Their state comes from the chain.
One call to Seaport moves your counter, which every signature you have made was signed against. Offers already filled are unaffected.
Offers posted on chain
Seaport keeps an offer's terms in its own event log when a maker publishes one. This reads that log — there is no database anywhere — and shows the ones anyone may still take.