Do ARCA orders execute against market makers or other ECNs? I always thought that it did, but I often see ARCA locked with another ECN, usually INET. And they just sit there , unexecuted, with the bid and the ask the same price. I remember when the REDI ecn was still around, and there were two types of REDI orders, one that was proactive and would execute against anything, and another type that would only execute against other REDI orders. Is this what's going on with ARCA?