They are worn indoors and in Dojos with the Zori (sandals).
In Kyudo you wear white Tabis. Traditional, bowmen following Heki teachings usually don't use Tabi except if they are training together with Shomen archers. In Europe it's handled less strict: If you got cold feet, wear Tabi!
Our Tabis are at least enforced with an additional layer of cloth, and we do have a version with rubber nubs.
Measure the length of both your feet in cm. If one is longer than the other, use this size. Best, do this in the afternoon, standing on a paper while another person outlines your feet.
Please mark the notes in the product's description! Some Tabi mayshrink half a centimeter on first washing.