Streaming is the normal case. Recordings, pictures and the map come over the network while you use them, so an ordinary tour needs a mobile connection.
You can also take places with you. While your subscription is running, open a place and use Offline: its recording, its stories, the pictures and the piece of map around it are saved to your device. What you have saved is listed under Offline content in your profile, and you can remove it there or on the place itself. Downloads are for your own use — our Terms of Service put it the same way.
Saved does not mean frozen. If a guide corrects a recording or a text, your saved copy is quietly brought up to date the next time you open that place with a connection. Nothing is ever exchanged while you are listening.
Abroad and in flight mode, what you saved beforehand plays — for up to 30 days after the app last confirmed your subscription, so a long stretch without a network does not shut you out of what is already on your device. Past that the saved places stay where they are, but they only play again once you are signed in and the app can check. If it checks and your subscription has ended, it removes the saved places from your device — as do signing out and switching to another account. Anything you did not save needs a connection, and saving needs one too — so it is worth doing before you travel rather than after you land.
Stored on your device otherwise: your sign-in, your language settings and the list of your favourites and tours. That data takes almost no space. Everything Citily keeps on the device, saved places included, goes when you uninstall the app.
Data usage. A recording is a few minutes of speech and therefore much more frugal than music or video; the map loads tiles as you move. Over an hour of strolling through a city, usage stays in the range of a few dozen megabytes. Saving a place downloads those bytes once instead of every time you open it.