When navigating a journey, Beeline Velo will work offline.
That means that if you lose signal or want to save phone battery by using Airplane Mode, Beeline Velo will keep navigating you correctly along the route that you planned.
The only feature that will not work offline/without a data connection is re-routing.
To plan a route in the app your phone does need to be online (have a wifi or mobile data connection).
To import a GPX file in route mode you will also need an internet connection
However, you can import a GPX file in compass mode without an internet connection, and start navigating as usual. This is because compass mode only uses GPS and doesn't need to plot a new route. If you go off-route in this mode, your Beeline will point you to the next waypoint.