Leaving aside "flooding the order book"; the primary difference is that the HFT orders are made in good faith with intent to trade at the order price based on t

Leaving aside "flooding the order book"; the primary difference is that the HFT orders are made in good faith with intent to trade at the order price based on the current market. Layering and spoofing are not done in good faith: there is never any intent to trade, just to convey a false appearance of market demand.