Go to "Sales -> Configuration -> Settings" and scroll down to the "Quotation & Orders" section. Enable the "Partial Confirmation" checkbox. Enable the "Save Unconfirmed Items" checkbox if you want to create a quotation with all the unconfirmed items. Define a suffix for the unconfirmed items quotation in the "Unconfirmed Quotation Suffix" field. If no suffix is defined then a standard "-U" suffix will be used. Eg "SO00023-U".