Open any booking in the diary and press Message. The conversation opens pinned to that booking, with quick replies for the things a shop says a dozen times a day ("Running about 10 minutes late", "See you soon", "Bring your card") and a box for anything else.
The customer sees your message on their booking page and gets one email with the text and a Reply link. Their replies, and their own quick replies ("On my way", "Running a bit late"), appear in your inbox under Messages, live on the screen and with an unread count on your phone's tab bar. You get an email nudge too, which you can switch off under Settings, Features.
Shared notes go the same way: on a booking, type an aftercare note or what to bring and press Send note, and the customer can see it. The private note underneath it never leaves your diary: it lands as a dated entry on the customer's profile (press their name on any booking), alongside their bookings, spend, no-shows and your conversation.
Messages are on from the start for trades that live by them (trainers, instructors, clinics, studios, home trades) and off for others; switch either way under Settings, Features. Off hides the inbox and the doors; nothing is deleted. Messages are included on every plan.