Each request carries two labels.
How it reached you
The Any source filter on Inbox lists these: Online booking, Website form, Quote request, Referral, Added by you, Missed call, Text message, Customer page, Google, Plan sign-up and Commercial request.
Where they found you
The Found you anywhere filter lists these: Google, Direct, Referral, Social, Other sites, Printed card and Your follow-ups. A request with none of these shows no label.
See it on a request
Open the request. Where it came from shows Request type, Location, Found you through, Page, Status and First reply.
For requests you add yourself
When you press Add request, choose How did they reach you? and Where did they find you?. Choose Not sure if you don't know. Ask new customers how they found you: it's the best data you'll get.
Totals
Insights counts requests by where they come from and shows the job value by where it came from. See Reports and the monthly report.