The raw respiration signal has arbitrary units, the mL values are calculated on the TidalVolume and MinuteVentilation only (please refer to this link).
We do a general calibration using the weight and height of the user, but it is not a complete calibration per se. The calculated coefficient is the conversion factor to convert the raw data into an adjusted value in litres. You can change that coefficient to fit any calibration using an API call. We plan to revisit this calibration method. Calibration may require an external device for a real gold standard reference, such as a spirometer. To learn how please refer to this link.