Encryption in transit
Exchanges between your application and the API pass through encrypted channels. This applies to both reading and writing resources: whether you are retrieving a series of measurements or writing a new observation, the transport is protected. OAuth tokens do not circulate in plain text. Encryption in transit protects data during transport; it says nothing about what happens once the data is written.
Point of vigilance: your side of the integration must also respect this contract. A client that caches measurements in plain text on a shared workstation cancels out part of the benefit.