What's the difference between “QuickNav” and “QuickNav First Letter Keys” when using a braille display?

Hi, I am using in ioS and Brailliant BI 40 X Braille Display.
I will be happy to know the diferents between two things: "Quick Nav" and "Quick nav first Letter Keys"? I don't sure how it translated to English.
I mean to the commands: q+space+7dots.
Thanks.