For those of us who are Jeppesen users, the reference can be found at Jeppesen/ATC/AU-936.
when a VFR aircraft, or an IFR aircraft cleared for a visual departure is issued heading instructions
13. {symbol} (instructions) RUNWAY HEADING [LEFT (or RIGHT) HEADING (degrees) VISUAL, CLEARED FOR TAKE-OFF
The {symbol} which I couldn't paste into the above means:
Unique to Australia (ICAO silent)
Jeppesen/ATC/AU-920 refers.