This is the one setting that changes how well the rest of usnapit works. Your calorie target stops being a formula and starts being a measurement - and the two things that takes, your weight and how much you moved, are already being recorded by your phone or your watch. Connecting lets usnapit read them instead of asking you to type them.
Step on a connected scale, or weigh in anywhere that writes to Apple Health or Health Connect, and it is in usnapit before you open it. A trend line needs regular points to be any use, and the usual reason one falls apart is simply forgetting to type the number in.
Active energy comes across from your watch or phone, and usnapit averages the last seven days of it. Deliberately an average, not a daily bonus: a target that jumped every time you moved would be telling you to earn your food, and it would be a different number at breakfast than at bedtime. A hard week raises the following week's budget, steadily.
A weight you type into usnapit is written back, so your health app and everything reading it stay in step and you are not entering the same number twice. Nothing else is written - usnapit does not push your meals into your health app.
During setup, or later under You › Activity. One toggle - iPhone asks for Apple Health, Android for Health Connect.
Your phone grants each permission separately, and you can change or revoke any of them there at any time.
Weight and activity arrive on their own, and your calorie target quietly gets more accurate.
Health data stays between your device and your own health app. usnapit works without it - you can type your weight in and skip the whole thing - it just has less to learn from.