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GUI for .NET Primer
Basic Types of Forms and Application Design : Form and window coexistence

Form and window coexistence

To allow forms and windows to coexist in an application, several ABL enhancements work to tie them together. A FormProxy object is a wrapper that is created for every ABL window. Similar to a form, the FormProxy allows the window to reside on the form chain, which is a list of all running forms.
Except for the default window, a FormProxy is created for windows that are explicitly created in the session, not for windows created internally by the AVM. The AVM maintains form proxies, automatically creating and destroying them. Even though you do not work with FormProxy objects directly, it is helpful to know about them, since they exist on the form chain.
A shadow window widget, which is created for every form, provides a means of parenting a window to a form. The shadow window has no visualization.