The way to learn any programming language is to take a class, not read some code and post a few questions.
CMROLL is a routine that creates a delay in processing. The parameter for CMROLL must be an I2 variable. The value of the parameter is the time in seconds you wish to delay execution. It can be used, for example, to create progress bar charts.
The USR routines are written by, and maintained by, and enhanced (usually by new versions) by Software AG. Suppose you write several programs to work with Natural source code in the editor. Then, in the next version of Natural, some of the control characters and formats are changed. Your code is suddenly worthless. By contrast, if you use a USR that has the same functionality, it will either be modified, by Software AG, to reflect the new format, or be replaced by a new USR with the same or similar functionality.