Flank steak is typically cited in recipes for Chinese stir-frys, but I usually use shoulder cuts and the tenderer parts of the round. Beef terminology is so thoroughly confusing and changes so rapidly from one area of the country to the next and from year to year in the trade that I rely mostly on my eyes and the advice of the butcher when I am choosing one cut from another. Flank steak is good when it is very fresh and meticulously trimmed and when the recipe calls for you to slice it on the diagonal and marinate it prior to stir-frying—all of which ameliorates its native tendency to toughness. Top round is better, however, if you can afford it, and I personally find that the time saved in trimming is alone worth the extra price.