An EPL file is a file that contains the source text for an optional package specification and one or more event declarations or monitor definitions. A file can consist entirely of event declarations without any monitors.

A monitor is a group of related variable declarations and actions. The monitor is the primary unit of execution in EPL. An action is a group of related variable declarations and statements. An action can either be part of a monitor or part of an event declaration.

The executable statements (except for global variable intializers) are always inside an action. An action can be either a subprogram or a function. The difference is that a function has a return value and a subprogram does not.

Each file is injected in either whole or not at all; if some parts compile validly but others do not, nothing is injected and an error is returned. Injecting can also return warnings about the code injected. For example, use of keywords that may be reserved in the future.