What is a sub box?

Sub-panel?

Why do you ask?

More detailed questions get better answers.

A box to put submarines in? I think they are available for sale in Groton, Conn. And Charleston, S.C. :)

If you are speaking of a "Sub Panel", then by definition there is no such thing. It's a common reference to a secondary electrical panelboard.

Code does not define anything as a subpanel.

But basically, You have the Main Panel Board where your electrical services enter the residence.

What you are calling a sub panel is actually another panelboard that is fed off the main panelboard.