I’ve actually never sent a SMART order to any (spread or outright) futures market before, but for FOPs it functions in a way where you cannot go in-between the bid and offer while having the order registered in the native exchange order book.
Since all FOPs have greater than 1 tick wide bid-offer spreads, ES is the lowest at 2 ticks during prime hours and other commodities being always (sometimes much) wider, the SMART method will not allow you to attempt price improvement/discovery as it will only hit the current posted bid or offer.
And this is particularly stupid given that hitting the mid-point is basically *normal* in FOPs. There are quite a few times I've been insta-filled right at midpoint, so for IB to hold back smart orders because they're not hitting the bid/ask is stupid. I mean what's the point of that anyway? What possible purpose does SMART routing have here when "marketability" has nothing to do with whether I hit the bid or lift the offer. On top of that, not hitting either means I'm providing liquidity and not taking it.