What part of beef is used in beef chow mein?
Chow mein is a Chinese dish of stir fried noodles.  While popular throughout Chinese cuisine, I am guessing that you are referring to the dish found at Chinese restaurants throughout India, the UK, or the US.  In these restaurants, a protein such as chicken, beef, or seafood is often added.  Remember, these are high heat, fast-cooked dishes, most often cooked in a wok.  So, the proteins must be cut small enough to cook quickly, and lean enough to result in a tasty and tender bite.  There is not time to render fat or break down collagen when stir frying.  So when considering beef, pork, or lamb specifically, cuts that lend themselves to quick cooking are chosen.  For beef, I would say sirloin or flank are two good options.  In the end, this ingredient choice is mostly about cooking method.