I can’t find anything on this with just a quick search.
And this looks like a good link with more info:
https://www.michiganfreedejurestate.us/files/HISTORY_OF_THE_US_CIVIL_FLAG.pdf
The United States Civil Flag of Peacetime We the People of the United States, actually have two national flags, a military flag and a civil flag for peacetime. They have several important distinctions and meanings.
Just found that one, looks like a wealth of info there...