What is your Natural environment - Mainframe, Unix, Windows, NaturalONE?
Here’s a link to the Unix documentation for Tools and Utilities. See the Chapter for SYSEXT.
SYSEXT is a utility/menu/library/directory that contains or lists all of the USRnnnn API modules. Most are subprograms like USR1058N, and each should have corresponding Text object like USR1058T that describe the purpose and parameters, and a Program source example of how to set up the parameters and call the routine, like USR1058P.
Hope that helps.