The Method Library functionality allows you to add one or more include files into the generated code. The include file is inserted after all the definitions, and before any procedures and functions. Thus, allowing you to use include files that references the definitions or uses the preprocessors generated by the AppBuilder.
The term Method Libraries originally refers to the special type of include files that contains the methods for the various type of components of the ADM1 (Application Development Model) framework. The method libraries provide code reuse and ensure that each instance of an ADM1 class have the same logic being compiled into the r-code.
The ADM2 also uses the method libraries, but in this case the reusable code and class hierarchy are implemented in super-procedures. The method library defines the class of the instance, but contain very few methods, instead they initiate and bootstrap the class super procedure and super procedure hierarchy to the instance. They also include some variable definitions and preprocessors directives to control what to include in the compilation unit. But, the preprocessors generated by the GUI Designer are rarely used as compiler directives, but typically passed to the super procedures as property values.