GoBe3 is designed to be worn on a wrist, as a watch. You can wear it on either arm.
As for ankle use, it will be difficult to even attach the band to your ankle. We don't offer ankle straps and we also cannot guarantee accurate readings when work on the ankle. So if you do it, it will be at your own risk.
If you have any questions, please email us at firstname.lastname@example.org We will be happy to help you!