Regarding Mid Grade Spirit Stones, Yang Fu had a long conversation with the disciple from the Task Pavilion and learned some unwritten rules within the Cloud Hidden Spirit Sect.
Cultivators above the Foundation Establishment Stage, when cultivating, should ideally use Mid Grade Spirit Stones, which can enhance their perception of Heaven and Earth Spiritual Power, better integrate the Spiritual Energy of heaven and earth into their magic, and far surpass the effectiveness of Qi Refining Stage Cultivators who merely rely on their own internal Spiritual Power to cast spells.
Of course, if there are no Mid Grade Spirit Stones available, one can absorb Low Grade Spirit Stones for cultivation, but the speed and quality are far inferior to that of Mid Grade Spirit Stones.
Some poorer Foundation Establishment Stage Cultivators use Low Grade Spirit Stones, but most within the Cloud Hidden Spirit Sect use Mid Grade Spirit Stones for their cultivation.