I never really understood the "with delivery in Cushing, Oklahoma" thing, and the Delivery section on page 3 doesn't make it too much clearer.

Surely there are people trading in these contracts that... don't want their oil delivered to Cushing? The Delivery section makes it sound like maybe it can be delivered somewhere else if the buyer and seller agree, maybe?

And Wikipedia does make it sound like Cushing really can be a bottleneck: https://en.wikipedia.org/wiki/Oil_industry_in_Cushing,_Oklah... But... how? It seems like such a bizarre setup to literally require all the oil to come to this one specific town, I assume I'm missing something obvious?

This level of standardisation is indeed what makes them so liquid and useful!