A position holder is the company that owns fuel while it is sitting in a terminal. Because it holds that fuel, it is generally the one who owes the tax when the fuel crosses the rack onto a truck.
A terminal is the large storage site that pipelines and barges feed into. The fuel stored there is owned by someone, even though it sits in shared tanks. The party that owns a given lot of fuel in the terminal is called the position holder, because it holds a position in that fuel.
The title matters mostly for tax. The per-gallon fuel tax is usually charged at the moment fuel crosses the rack, the loading point where a truck fills up. The position holder is generally the one on the hook for that tax when it crosses, and it then passes the cost down the chain.
Knowing who the position holder is on a given load tells you who is responsible for the tax and who is selling the fuel into the chain. For a jobber buying at the rack, the position holder is often the supplier behind the deal.
In useThe supplier is the position holder on the fuel in those terminal tanks, so it owes the tax the moment the jobber loads a truck across the rack.
See also Above the rack, Rack, Motor fuel excise tax