I have this code and wonder what the dash dash signifies
proc sort nodupekey; by mem_id - - prov_eff_dt;
SAS Variable List
It means to do the sort according to all the variables from mem_id to prov_eff_dt in the list of variables of the dataset that is getting sorted.
It signifies a list of variables beginning with mem_id and ending with prov_eff_dt based on the order the variables were defined in the dataset. Here is the section that covers variable lists.