I expect it depends on what the stop turns into when elected.
If it's a plain stop (i.e. stop market), it will most likely remove liquidity, since it becomes a market order that will get routed to whoever is displaying the best auto-ex bid/offer.
If it's a stop-limit, and the resulting order is marketable, it's the same as the stop-market scenario. If it's not marketable, and gets posted to an ECN, it will add liquidity when/if someone hits/takes it.
If it's a plain stop (i.e. stop market), it will most likely remove liquidity, since it becomes a market order that will get routed to whoever is displaying the best auto-ex bid/offer.
If it's a stop-limit, and the resulting order is marketable, it's the same as the stop-market scenario. If it's not marketable, and gets posted to an ECN, it will add liquidity when/if someone hits/takes it.