Positioning.
Hi Beechgal, welcome to Pprune.
When a crew member "positions" that means he is riding on an aircraft (either in the cockpit jumpseat or in the passenger cabin) to "get in position" for his next duty assignment.
He's not technically a part of the "operating" crew *but* he is considered "on duty".
Naturally, if one is positioning to operate, he's obligated to observe all rest requirements and abstain from alcohol.
If he's deadheading (just riding along, perhaps after operating) his rest period will begin usually 30 mins after the flight blocks in.