Cloud Sync keeps your ATOM the same on every device you sign in on.
What syncs: everything. Legs, shows, expenses, settlements, buyouts, crew, bus bids, hotels, per diem, budget, W-9, contacts.
When: ATOM pulls your data when you sign in and saves changes as you make them.
Several devices at once: you can be signed in on your phone, your computer and the Mac app at the same time. Signing in on a new device does not sign out the others. A device stays signed in until you sign out there, or until it goes 60 days without being used. ATOM keeps up to 30 signed-in devices per login; past that, the one used longest ago is signed out.
Signed out everywhere else: turning two-step sign-in on or off signs out your other devices.