Big Q or other AS400 knowledgeable,

My company makes a custom Win32 App for golfclubs and one of our major accounts which covers several clubs is going to drop our software since they are purchasing an AS400. Don't know what the specs are on it or really what an AS400 is but their administrator claims that there are some windows apps that will run on it. Is this true and what requirements does a windows app need to meet to run on an AS400? POSIX compatible, I am clueless on this one

Thanks a whole lot for any reponses in advance,
tony