Sorry to have bothered some people with a question that apparently made no sense them. Sorry also for not having searched deeply enough on the CME website. Strangely enough only on a third look I found some quite interesting documents. In case someone stumbles about the same question sometime here is a quite good description of the algorithms used (or in case you want to learn what an algorithm is and computers at exchanges need one to work): http://www.cme.com/files/cbotclientimpact.pdf page 55 cont. also (less detailed) http://www.cme.com/trading/get/abt/functionality/matching.html