ZPL provides a special class of variables known as config variables, or config parameters. These are essentially write-once, base type, variables that can be specified at load time via command-line parameters.
There are three ways to initialize config variables. They may be specified at runtime by using the -s flag on the command line (see Section 9.2), they may be initialized in a file by using the -f flag on the command line, or they may be given default values when they're declared in the text of the program. The values specified at the command line override any other values, and those given in a file override any of the default values. Config parameters may be initialized in terms of expressions of constants or other config parameters.