It is when you talk about the petrodollar as something important globally. It definitely is not something you could call a system.

Here is wikipedia for a start: https://en.wikipedia.org/wiki/Petrodollar_recycling

“In reality, the use of dollars in international oil transactions increases overall U.S. dollar demand by only a tiny fraction, and the dollar's overall status as the major international reserve currency has relatively few tangible benefits for the United States economy as well as some drawbacks.[41][42]”