Imagine everything as as three buckets.
Your first bucket is daily allowance, this you empty first when you enter food into the diary and is refilled each day.
The second bucket is exercise, you fill this up when you exercise and add it to the diary. This then dependent on mode anything left in it is emptied on a daily or weekly basis.
Your third bucket is weekly, this is bonus pool that can be used gradually over the week or in one go, this is refilled once a week.
The app will automatically use your weekly and exercise values as soon as you go over your daily allowance.
What it uses first is dependent on how you configured the application on first run, so it will either use your exercise first (recommended) or your weekly allowance.