The cultivation at the Qi Refining stage mainly involves two things.
One is the constant refining and compressing of mana.
The other is the continuous expansion of the Dantian space.
The former is the crucial basis for the ten stages of Qi Refining. Each time the overall compression of mana is completed, one level is advanced.
After nine compressions, the gaseous mana is fully liquefied. This is the Stage X of Qi Refining.
At the same time, it's one of the thresholds to transition from Qi Refining to Foundation Establishment.
On the other hand, the latter occurs naturally while cultivators refine mana.
Different cultivation methods lead to different sizes of Dantian space developed.
Theoretically, the bigger the Dantian space, the better because more space can store more mana. If engaging in a fight with others, it can give one an advantage.