I am new to US stocks and find that US stocks are distinguished by ticker symbols. For example, AAPL is for Apple Inc. hwoever, in CRSP and COMPUSTAT, there are CUSIP, PERMNO and PERMCO to distinguish one stock from another.
In the section "The playing ﬁeld" of Fama and French (2015), it's said "the sample is all NYSE, AMEX, and NASDAQ stocks on both CRSP and Compustat with share codes 10 or 11". I wonder what does "share codes 10 or 11" mean? For CUSIP or PERMNO? Or something else? It may be a silly question, but I really need help. Thanks.