Which certificate do you mean? There are two types of SSL certificates you can deal with:
The server certificate
This is standard SSL, the server sends its certificate to the client which must have access to the corresponding root certificate in order to authenticate the server. If the client does not have access to the root certificate it will either raise an error/warning (web browser) or reject the connection alltogether (Progress/OpenEdge GUI and WebClient).
The client certificate
The client certificate is not support by many software vendors - one of 'em is Progress. But, if you are running WebSpeed then the client certificate is sent by the browser and authenticated by the WebServer - therefore WebSpeed and Progress as a vendor is not part of the SSL handshake.
HTH, RealHeavyDude.