Thanks for the answers guys...
I am familiar with QFE and QNH but the climb 300 doesn't really make much sense.
Assuming it means to climb to 30000ft, that seems like a looooonnng time to maintain runway heading, especially as most missions occur at or around 10000ft.
If it means 300ft then perhaps we can assume we are allowed to deviate off runway heading at 300ft. Which seems more plausible, but still seems fairly low... correct me if I am wrong but in actual airports they usually make you climb to pattern altitude before departing VFR.
Perhaps it is an indication of minimum safe altitude around the airport.