These levels are more like “Intensity” levels so once you are comfortable with working at the V.Light intensity and/or your improvement appears to flat line with little or no improvement, then you Manually increase the intensity of your work out to Medium for example.
Each time you use your POWERbreathe, your first two breaths will calculate your optimised working level (Auto-optimising feature). This will step incrementally over the next three breaths to your training load for the session, and is based on the level of intensity you have manually selected.
The most important message is to ensure that you can maintain your 30 breaths for the session, if the Level or Intensity is too high then you will find that the session will be short and you are training to fail. Consider as a comparison: Weight lifting as an example - the incomplete Arm curl or doing half completed repetitions within a set - It is more important to complete those 30 reps.