There are a number of stock member names which are accessed via this macro. The text of such a stock string is put as the argument to the macro such as SE_STOCK(length) or SE_STOCK(this). For a particular string, such as length, SE_STOCK(length) is equivalent to SE_MEM(length). The advantage to using the SE_STOCK macro it that the macro is faster. However, only some of the more commonly used member names are available with SE_STOCK.
All of the stock strings may be found in the file src/misc/stockstr.h