What does volume outside my top pillars mean?

This is the collective volume outside your two pillars with the most volume.
(Example: If I have 4 pillars. Pillar 1 has 500 CV, Pillar 2 has 450 CV, Pillar 3 has 600 CV and Pillar 4 has 300 CV. Pillar 1 and 3 are my top two pillars. The volume in pillars 2 and 4 are my volume outside pillars 1 and 3.) Your personal volume (PV) also counts towards your outside volume.