Every client you've quoted or billed lives in one list, with their full history attached.
Go to Clients → Add Client and fill in their details. Only the name is required — phone and email are both optional, so you can add a client even if you only have their name for now and fill in the rest later. You can also add a client on the fly while creating a proposal — there's no need to add them separately first.
Opening a client shows every proposal and invoice connected to them, so you can see their full history — what was quoted, what was accepted, what's been paid — without digging through separate lists.
Use the search box on the Clients page to find someone by name, phone, or email. This is the fastest way to jump into an existing client when starting a new proposal for a returning one.
Open a client and update their name, phone, or email anytime — this doesn't change anything on proposals or invoices already sent, which keep the contact details as they were at the time.