A draft invoice doesn't get an invoice number and is not included as revenue in the reports. It's a fully non-binding invoice, just a draft.
To save an invoice as a draft click on the button in the upper right corner of your first cash register window.
You can find an overview of the invoice drafts by clicking on the Invoices tab in the Cash Register. You can then apply a filter to only see the invoice drafts.
By clicking on the invoice (anywhere in the line) a new window will open with its details. If you click on the button in the upper right corner, you can open the invoice as a PDF file.
You can either download or print this PDF file. This way you can use the invoice draft as a quotation. An invoice draft can always be edited or deleted afterwards, which wouldn't cause any problems for your invoice numbering (because it's a draft, it doesn't have an invoice number yet).
You can add additional information to the invoice (quotation) by adding a note to the invoice. This can be done in the first window within the cash register: