假定一变量的长度为60，而你定义的宽度只有30的话，很有可能会出现值截断现象，加上flow以后就不会出现，它会自动换行显示的

FLOW wraps the value of a character variable in its column. The FLOW option honours the
split character. If the text contains no split character, PROC REPORT tries to split text at a
blank.