G'day Ravan,
When an ATC gives a heading instruction, the controller is assuming both navigation and terrain clearace responsibilities from the pilot. MATS says:
3.2.6.6 An aircraft being radar vectored in VMC by day may be permitted to
arrange its own terrain clearance, provided that the responsibility is specifically
assigned to the pilot using the phrase “DESCEND TO (level) / CLIMB TO (level) /
TURN RIGHT / LEFT HEADING (degrees) / VISUAL”.
Even a VFR aircraft has to be told to turn/climb/descend visually if the controller is vectoring them below the radar LASLT. Another example is if you are being vectored off the runway on departure, the TWR will say "Turn left heading XXX visual, cleared for take off" as ATC is taking responsibility for terrain clearance by vectoring you, and then handing it back to you.
Sounds confusing, but if the controller says VISUAL, the just read it back and be done with it. It generally does mean something. I'm sure AIP has a similar explanation.
Cheers,
NFR.