How does the respiration sensor work and what kind of values does it measure?

The NeXus respiration (RSP) sensor uses a magnet with a hall sensor (magnetic field sensor) to measure the stretching of the band.

The measured value is the raw data from the sensor and is related to the expansion of the chest belt. The RSP rate (calculated by the software, in breaths per minute (BPM)) is derived from the raw data.

Every time you place the chest belt, you place it differently. That is why the absolute values are different. The RSP measurement is not a measurement for volume of inhalation/exhalation. For example: with bigger patients the difference of the signal will be relative small because the circumference of the body changes relatively slow.
Therefore these absolute/raw RSP values cannot be compared with RSP values of other sessions, or other clients. However, the RSP rate values can be used for comparison.