Large parts of the physical oil market have no exchange. Deals are bilateral and private, so there is no order book to read a price off. What fills the gap is price reporting agencies: firms that collect bids, offers and completed deals from participants, apply a published methodology, and publish a daily assessment of where the market was.
This is not a computed average of everything reported. Methodologies specify a time window, define which deals are eligible for inclusion, normalise for cargo size and loading dates, and give the assessor discretion where the evidence is thin. The output is a judgement constrained by a documented procedure, which is a genuinely useful thing and is also not the same object as a settlement price.
Two properties matter when you use one. First, participation is voluntary, so what the agency sees is a sample, not a census, and the sample is self-selected by parties with an interest in the resulting number. Methodologies exist largely to constrain that incentive. Second, on a quiet day there may be little or nothing to assess, and the published number then reflects the assessor's read rather than observed trade.
Assessed prices nonetheless carry enormous weight, because they are written into contracts. Dated Brent underlies a large share of the world's waterborne crude trade, and it is an assessment, not a settlement. Its methodology has been changed several times, most consequentially by admitting non-North-Sea grades into the basket to address declining local production, and every change altered what a long history of the series means.