The SSL client which can be found in Management Console under Configuration > SSL contains the settings needed to make an SSL connection.
This profile can be used by any HTTP or HTTPS proxy service that needs to make an upstream SSL connection

The SSL Client is NOT, as might be thought, used by the SSL forward proxy (in other words when intercepting SSL traffic). It is only used when the ProxySG is acting as an SSL client, for example when it is set up as a reverse proxy or when the ProxySG rewrites a URL from http:// on the client side to https:// on the server side. Under these circumstances the ProxySG will use the settings found in SSL client to set up the session.

At present, there is no way to restrict what ciphers the ProxySG will use when completing a handshake with the client.