The baby sets we have are best for a baby sitting up on their own. We used ours when baby started sitting up at the table with us - from 6 or 7 months. Harness helps to keep them safe and upright.
Til then we used the Tripp Trapp baby basket. Really best to take advice from Stokke website or supplier. We found [www.backinaction.co.uk
We used the baby set from 6 months to 2.5 years, and harness from 6 months to 1.5 years, but everyone's needs are a bit different.