What traffic is sent over the build-in VPN connection? Under System Preferences &gt; Network &gt; VPN &gt; Advanced &gt; Options I can choose to "Send all traffic over VPN connection".

Contrariwise, this would mean that if this option is not enabled (which is default) some traffic is treated as if no VPN connection was established: e.g. The IP address when surfing is still the same.

What traffic is (not) sent over VPN when this option is disabled?
This really depends on the routes your VPN server is pushing when you connect. It could push only routes to private networks or it could push a route for the default gateway.