In the topic that describes a given EGL system function, a syntax diagram gives you details on the type of each function parameter and on the type of value returned, if any. The name of the function library is specified early in the topic.
StrLib.clip(text STRING in) returns (result STRING)
If the parameter specification is surrounded by brackets ([ ]), the argument associated with that parameter is optional. If the specification is surrounded by braces ({ }), the argument is also optional, but in this case you can include multiple arguments that are all of the same type.
If the function returns a value, the diagram shows the word Returns and a parenthesized name and type. The topic refers to that name when describing the return value, but the name is otherwise meaningless.
If a returns clause is surrounded by brackets ([ ]), the return value is optional.