Hi Nina, and welcome.
I also chose "Mostly seated with some movement." The program then says I need about 2200 calories. But I think the program is assuming we're seated with some movement 24 hours. So every day I enter sleep hours. I just keep it at 7 hours - that way every day I just go to "Recent activities" and choose sleep. That drops my calories down to about 1900, which is about right for me. The 1900 calories is how many I would need to consume to maintain my current weight. Or put another way, my body burns 1900 calories a day sitting and sleeping.
Yes, any extra activity you do is a bonus.
When I do anything other than sit for any amount of time, I enter it in the activity log (working in the yard, walking, calisthenics, washing the car, etc.).
If I eat 1400 calories a day (that's 500 less than what my body burns daily), that's 3500 calories a week less that the amount needed to maintain (3500 calories = 1 lb). If my non-seated activities for the day total 300 calories, then that day my body needed 2200 calories to maintain current weight. If I ate 1400 calories, that's 800 calories burned - almost a 1/4 lb!
Age 61, Height 5'9"
March 2010 145 lbs, Jan 2013 173 lbs
1st Goal 145 lbs 5/25
New goal 140