Τhe series of books for learning the 6 string Bass Guitar with the mode system concept is based on the processing and the geometrical structure and coexistence of the modes in every key.  The system begins with the mother scales, the principal scales that exist in the western musical system.  Those are the major scale, the harmonic minor scale, and the melodic minor scale.  We do not mention the natural minor scale as a mother scale because its construction is covered during the studies of the major scale in which we find the natural minor as its 6th mode, known as the Aeolian.
When we look at each mother scale, we find the modes that are built on each scale degree, and we study them through a specific shape pattern, harmonization, and most importantly specific fingering positions. The most essential point is that with this system we have to maintain a very specific fingering pattern for each mode, so that the movement becomes consistent and automatic through our fingers but also our brain, which leads to flexible and effortless playing.