"It is the largest N US listings, which is what an index of that shape approximates too. Never called 'the S&P 500'."
The temptation after the approximation is accepted
Having agreed to compute the measure from available data rather than licensed data, there is a second decision immediately behind it, and it is easy to get wrong because it feels like a labeling detail.
What do you call it? Everybody knows the famous index. Calling it "S&P 500 concentration" would be instantly understood, would look professional, and would be a small lie that costs nothing today.
The refusal is explicit in the code for this measure: it is never called that. The measure is stated as what it is — the top ten companies as a share of the largest N US common stocks by market cap — and the borrowed name is left on the shelf.
Scope that sentence carefully, because the same repository has a counterexample one module over. A composite indicator built on the same broad-market ETF proxy is named "S&P 500 P/E", and that name is served in the API payload and rendered on the dashboard. So this is a discipline the product holds in the place it was argued about and has not yet applied everywhere — which is worth knowing, because it is the honest state of most such rules. A naming discipline is not adopted the day it is written; it is adopted one grep at a time.
What a borrowed name silently imports
The reason is not pedantry. A famous name carries a specification, and adopting the name adopts claims your computation does not satisfy.
A real index has an explicit membership rule, a committee that applies it, an eligibility test, defined treatment of dual listings and foreign domiciles, a published rebalancing schedule, and a float-adjusted weighting method. "The largest 500 by market cap" has none of that. The two sets overlap heavily and are genuinely not the same set, and their differences are exactly the kind that show up in a concentration number.
So a reader who sees the famous name reasonably concludes they can compare the figure against a published one from a data vendor. They cannot. When the numbers differ, they will assume one of the two is broken — and the actual answer, that these are different measures wearing one name, is the only answer nobody will consider.
Where the description lives
The definition does not live only in documentation. It rides in the source string of every stored row, spelled out with the actual counts and the date the measurement describes.
That placement is the same principle as carrying a percentile's window on its row, applied to a definition rather than a caveat. A row extracted into a spreadsheet, quoted in a delegation brief, or read by a future consumer arrives carrying its own definition. Nobody has to have been present for the naming discussion to know what they are looking at.