
Explanation:
Box 1: HTTP request header
If you are using ASP.NET and configure your app to use client certificate authentication, the certificate will be available through the HttpRequest.ClientCertificate property.
Box 2: Base64
For other application stacks, the client cert will be available in your app through a base64 encoded value in the "X-ARR-ClientCert" request header. Your application can create a certificate from this value and then use it for authentication and authorization purposes in your application.
Reference:
https://docs.microsoft.com/en-us/azure/app-service/app-service-web-configure-tls-mutual-auth