01 · Notes
Each account’s Fastmail Notes are under that account in Mailboxes. Read, write, pin and delete them; they are the same notes you see on the Fastmail web app.
02 · Calendars
Switch on Calendars in the phone’s calendar for an account in Settings → Mail accounts. Its Fastmail calendars appear in Android’s calendar, so Google Calendar, or any calendar app, shows them. Changes go both ways: an event you add or edit on the phone goes to Fastmail, and changes on Fastmail come to the phone.
Switch it off and the calendars leave the phone. Nothing is deleted on Fastmail.