When setting the rules for Quote Expiration, you are able to choose between "Marking a Project as Lost" or "Disabling Online Signatures" at the time the quote expires.

When a project is marked as "Lost" (Article), it is removed from your "Open" projects list and automatically archived. This means inventory on this expired quote will no longer participate in conflicts. Your client cannot sign a lost quote, and may no longer be able to see pricing on the quote. They also will not be able to view all the items that they requested. You can unmark a project as lost here.

Disabling online signatures keeps the project in a "Quote Sent", or blue, status. Your clients will be able to view all inventory requested, but will not be able to sign the quote or pay online. You can re-enable online signatures for a specific project here.