Understanding 'checkbox' Add. function left of filename.

There is a data field immediately to the left of the >>>> File: name area <<<<< that has a checkmark box and the word Add.
What is the purpose of this data field? Do you ' Add.' the filename before starting the recording? What is the difference between having the checkbox highlighted and not highlighted?

I have not found any User Help files outside of the .html files generated when either the Timer information box or the Filename Format information box is ticked .

Thank you and the app is very appreciated.

Ed DeLauter