A User Defined Application Object is a global object of your own creation typically used to encapsulate all the logic you need access across the application. This may include Event definitions, Property definitions, common Routines for accessing messages or anything else which you need to remember or use repeatedly while running your Web Application.
Typically, User Defined Application Objects are defined as non-visual components of type PRIM_OBJT. For simplicity it is recommended is to have a single user defined application object.
Define_Com Class(#MYAPPOBJ) Name(#Application) Scope(*APPLICATION)
Of course you do not have to have a User Defined Application Object but it does make life a lot easier when you are navigating between different Web Pages and Reusable Parts and need to have access to common values and routines. Look at the Samples and Templates available with Visual LANSA for some implementation ideas. Also refer to Utilize an Application Object.