data monthly; /* You are creating an output data set named MONTHLY; This is a temporary data set and will be stored in the WORK library; It gets deleted once you terminate your sas session */   set test.monthly_exch_mv(where=(_type_=3)); /* #1 test.monthly_exch_mv is your source data set.  You applied a filter where you want the records to be _type_=3 only #TEST - is a permanent library which is assigned in the libname statement A libname statement is an alias to a path or a folder location; Once you terminate your sas session the dataset monthly_exch_mv will still be there "physically" (as stated in the location defined in your libname TEST statement) */   by yyyymm exchange; /* I presume here the variables yyyymm and exchange were pre-sorted, serves as a grouping or classification variables but does not seem to have any effect in your succeeding statements below */ format numberofFirms comma8.0; /* format statement is used because you want the value to appear in max length of 8 with commas) drop _type_;  (you do not want to see this variable in your output dataset */ run; /* execute and termiantes the data step */ ... View more