Thanks first.officer for your reply.
The second link is very useful. And answer the question about outbound ambulance flights.
On the second link. Item 10 is the one relating to my above question. This flight is below 20 tonne but above 10 tonne. The people on board are staff of the company who owns the aircraft and are on business trip. But because they are flying commercially on a 3 letter C/s they need to have security screening.