Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide

Characters that modify commands

Any of three characters can be placed in front of a command to modify how the command is run:

-- (dash)
Turns off error checking for the command
@ (at sign)
Suppresses display of the command
! (exclamation point)
Executes the command for each dependent file

Spaces can separate the modifying character from the command. Any command on a separate line -- whether modified or not -- must be indented by one or more spaces or tabs.

You can use more than one character to modify a single command.

Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide