Each project can have a proposition for the client. If the option "Client can approve proposition" is selected for a project, a client can approve the proposition. Only Admins can reset an approval.
There is a "Reference code" you can optionally use to match the proposition with documents such as invoices outside the system.
If there is no proposition, the "Proposition" tab is not shown when you view a project.
When you view a project, there is a "Download proposition" link on the "Proposition" tab, which will download a PDF.