"Client Default" is a feature where you can decide to customize and modify a certain feature -
1. Use your details for your client tax form. Read more - Use tax agent as contact on all tax forms (this feature will apply only for newly created clients (feature released date: 24 Aug 2021)
2. Give security to your client tax forms via sending sms or filling the date of birth to proceed signing their forms. Read more - Authentication Method
Navigate to gear wheel, and select "Settings" then press "Client Defaults"