"By" is a fairly global concept in JMP. If you run the Distribution Platform, and specify a "By" variable it will generate a separate Distribution for each of the different values of the "By" columns. The same is for the Summarize() function. Here is a quote from the JSL Syntax Reference for the Summarize() function.
Help==>Books==>JSL Syntax Reference
"Using a BY variable changes the output from single values for each statistic to a list of values for each group in the BY variable."
Good documentation along with an example can also be fount in the Scripting Index
Help==>Scripting Index==>Functions==>Statistical==>Sumarize
or
Help==>Scripting Index.........search on Summarize
Jim