Many options are available for controlling the way your program is compiled. These options are specified using directives when you invoke the Compiler. You can enter Compiler directives individually on the command line, or you can create default directives which are then invoked as a group either automatically, or from the command line, or from within the source code.
For normal operation you need to know about and use very few directives. In the majority of cases you will use the default values.