@Rick: A 7th use for data _null_ is when using proc sort to ONLY output records that DON't have duplicates. e.g.:

data have;
set sashelp.class;
id=_n_;
output;
if name ne 'John' then output;
run;

proc sort data=have out=_null_ nouniquekey noequals UNIQUEOUT=want;
by id;
run;

There likely are other similar uses related to procs, but the above example is the first time I'd ever seen it used that way.