When you or your macro issue a command unknown to the editor, PDF searches for a CLIST or REXX exec with that name. If the editor finds the command, it is implicitly defines it as an edit macro.

Program macros can be implicitly defined by preceding the name of the macro with an exclamation point (!). Remember that the name must be 7 characters or less, excluding the exclamation point. Program macros are similar to ISPF dialogs in that they must be made available as load modules in either the ISPLLIB, STEPLIB, or LINKLST library. See Program macros for more information.