I need help understanding the Headers list.

What is the difference between:

• "Record Headers In List"
• "Record Headers Not In List"

It seems obvious.  But I do not know what is actually happening when I click either or.

I know LoadRunner handles standard headers.  We use custom ones, however.  The standard headers are in BOTH lists.  So which do I need to use if I want to retain the standard headers being used, but insert the custom headers that I want?