TradeCourse, The example assumes that there is one MM at the inside showing 500; ARCA's 500 share allocation matches (SuperSOES filling only at the inside (NBBO)). If greater size is at the inside, then size is allocated to match. ARCA does not queue residual size on SuperSOES waiting for an MM to appear on the inside and the queue ahead to clear. With a SuperSOES order life of 90 sec. (180 sec. from 9:30 to 9:33 AM), that could be a significant delay. Instead, ARCA routes the residual size via SNET to "chosen" MMs. In addition to ARCA's MM-reliability statistics, maybe there are certain beneficial MM-ARCA relationships now (?). I don't know, but the word from ARCA management was "chosen". So, with fast fill being the obective, the example 2,500 goes to a participant that is available at the instant of allocation (SNET to chosen MM) rather than waiting 90 to 180 sec. on SuperSOES for an MM to maybe appear at the inside. If the SNET MM does not fill, the wait is only 15 sec. before that partial is refreshed, and that is significantly less delay than the max. possible if delayed on SuperSOES. ARCA values SNET for oversize orders. It can also reach through the market to hit MMs outside the NBBO, so it is still very useful. Regarding the lack of routing details on the TradeArca site, I agree, but that has always been the case with ARCA and most other ECNs. Unless you really dig, a lot remains a mystery. To pull the curtain back a bit, I am currently updating an order routing table that provides detail for all routes; I'll post when it is up.