Some engines support run-time lists, which allow a grammar to indicate a list that will be supplied at run time. This provides fill-in-the-blank functionality for a grammar for information such as e-mail names, filenames, fonts, and other values that cannot be predicted when the grammar is compiled.
The syntax for using a run-time list is the same as for words as rules. If a list is not filled in at run time, the grammar should still work even though portions of it may be inaccessible.