Three or more consecutive price levels showing imbalances in the same direction — a stacked buying imbalance is three+ consecutive buying-imbalance levels, a stacked selling imbalance is the mirror. The cluster signals sustained institutional aggression across a price range rather than a one-off spike, and creates a structural zone that often acts as support (buying stacks) or resistance (selling stacks) on retest. Tight stops just beyond the zone make these the highest risk-to-reward setups in order flow trading.