The shard consumes itself in a way that cannot be restored, draining its own essence with every action it performs, whether minor or significant.
The shard is formed entirely from a fragment of a soul domain structure, a precise portion carved from the original foundation, and it has no ability whatsoever to gather more or replenish what it loses. Every single unit it expends is gone permanently, beyond recovery, neither the shard itself nor the main body possesses any method to reclaim it once it is spent.
For example...
Robin possessed around 200,000 soul units, or perhaps slightly more, at the time he created this shard, and he used 30,000 units to form it.
That alone represents roughly 15% of the total time, effort, and accumulated resources of a Great Soul Master, a portion that cannot be dismissed lightly under any circumstance.
