What's the difference between a quote and a draft invoice?

A very basic question this, I'm sure. I've just started using Clearbooks, after using FreeAgent. In FreeAgent I'd create an 'estimate', that I'd then convert to an invoice once agreed by a client. (99% of the time, I'm really creating an estimate so I can see my revenue pipeline, more than because I need sign-off). In Clearbooks, how do convert an estimate to an invoice? I can't see an obvious way. (And can't find answer in help docs) Or perhaps it's more sensible to create a 'draft invoice' instead? Help appreciated! Cheers, Nick