You can add more than one quote to a proposal, and this is a great way to offer a cross-sell or up-sell, or a good/better/best set of options.
When you have more than one quote on a proposal, the quotes will have checkboxes next to them to allow you to present which options you’ve preselected to the prospect, and to allow the prospect to make the final selections.
How does this compare to user-editable quantities?
Quotes allow you to package up a whole solution. With multiple quotes, you are giving people the ability to pick one or more solutions.
User-editable quantities on line items just impact that one part of the solution.
You can, of course, combine the two.