open val minimumServerProtocolVersion: Int
The minimum protocol version required from the server. This is equivalent to the node's platform version number. If this minimum version is not met, an exception will be thrown at startup. If you use features introduced in a later version, you can bump this to match the platform version you need and get an early check that runs before you do anything.
If you leave it at the default then things will work but attempting to use an RPC added in a version later than the server supports will throw UnsupportedOperationException.
The default value is whatever version of Corda this RPC library was shipped as a part of. Therefore if you use the RPC library from Corda 4, it will by default only connect to a node of version 4 or above.