"$" in column 7 is a compiler directive. You can perform conditional
compiles on whole sections.

"D" in Column 7 is used in conjunction with the the SOURCE COMPUTER
statement. You can say:

SOURCE COMPUTER. HP3000 WITH DEBUGGING MODE.

and then a line with a "D" in column 7 is interpreted as a statement.
When you take the WITH DEBUGGING MODE clause out, all lines with "D" in
column 7 are treated as comments. You can add a special debugging
section at the top of the program:

 DEBUGGING SECTION.
     USE FOR DEBUGGING ON ALL PROCEDURES.
 DEBUG-SHOW.
     DISPLAY DEBUG-ITEM.
 DEBUG-EXIT.
     EXIT.
 MAIN-PROCEDURE SECTION.
 REST-OF-PROGRAM.

Every time a paragraph boundary is passed, the paragraph name is printed
and why the boundary was crossed (PERFORM, FALL-THRU, and so on).