Guidance about widgets within widgets

The APG contains the following guidance labeled widgets within widgets. 
Theoretically, it appears logical on paper. However, I am not sure it has 
any practical application. Or, if there are scenarios where it applies, I 
am not sure I would agree with the guidance.

Can anyone think of a real-world example where the following guidance 
applies?

Widgets within widgets:  The general navigation model is for   a user to 
tab to a widget, interact with the controls in that widget and then   tab 
to move focus to the next widget in the tab order. By extension, when the  
construct of a widget contains another widget, tab will move focus to the  
contained widget because it is the next item in the tab order. This 
continues   down the layers of widgets until the last widget is reached. 
    For example: We   have two widgets A and B on a page. Widget A 
contains within it Widget C and   Widget C contains within it Widget D. 
When tabbing, focus would land on Widget   A, then another tab would focus 
on C and then another tab would focus on widget   D. Because D is the last 
widget in C and C is the last widget in A one more   tab would move focus 
to B. 

Matt King
IBM Senior Technical Staff Member
I/T Chief Accessibility Strategist
IBM BT/CIO - Global Workforce and Web Process Enablement 
Phone: (503) 578-2329, Tie line: 731-7398
mattking@us.ibm.com