I'm curious to know why HttpWebRequest.ClientCertificates is a collection?
As far as I know, only one client certificate can be used (Is this merely true?) in client authentication.
An example of an application uses multiple client certificates, please?
While you can use only one leaf certificate for authentication you might want or need to send additional intermediate certificates so that the peer can build the trust path to the locally trusted CA certificate. This is true for both server and client certificates.