You'll see Which habits should we keep? when you log in on a phone that already has habits, and your account has different ones. Nothing is changed until you choose.
Each option shows how many habits it has:
- This device: your account is replaced with the habits on this phone. Pick this if this phone has the history you care about.
- My account: this phone is replaced with the habits in your account. Pick this if you just set up a new phone and want your old history back.
- Combine both: everything is kept together. The same habit's check-offs are merged, and your badges from both sides are combined. Shields and settings stay as they are on this phone.
On the free plan, only one habit can be active. If combining leaves you with more, you'll see Which streak stays active? Pick one; the others move to your archive with their history intact. Nothing is quietly lost, and you can bring a habit back later from Profile › Archive.
Not sure? Combine both is the safest choice, since it keeps everything.
Once you pick, both your phone and your account use the result, and syncing carries on as usual.