There are examples given when you are choosing between the options:
Sedentary: I have a desk job and/or sit most of the day (secretary, computer technician)
Lightly Active: I stand a lot of the day (nurse, teacher)
So if you have a typical desk job, I'd pick the sedentary option.
Does that help?
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford
"No matter how slow you go, you are still lapping everybody on the couch."