The power of subspace is infinite, and subspace is "generous". If someone needs the power of subspace, it will give generously without stinginess.
But the premise of everything is that this "person" must be their own "person", that is, they must possess the essence of subspace.
And the essence of subspace can be understood as a kind of "authority". The amount of subspace power one is qualified to mobilize depends on the size of their authority.
As for the source of this "authority", it varies a great deal.
Spiritual Energy practitioners do not inherently possess the essence of subspace; fundamentally, they are uncontrollable channels—breaches through which subspace power leaks into the real world, a disaster for the real world. It wasn't until The Emperor established the Spirit Network that such power could be controlled.