You can lock a circuit/spare/space in a specific location (and therefore on a specific phase) in a panel.
When balancing a panel, you can balance a subset of circuits. You can lock some of these circuits in place and only rebalance the remaining (unlocked) circuits on the panel. When circuits are locked, the Rebalance Loads command balances only the unlocked slots; the locked slots remain in place.
When a slot is locked, the cells are shaded.