All-time high

The phrase sounds absolute and is not, because it is a property of a data series rather than of a market. Four choices decide the answer: how far back the record runs, which venue the prices come from, which currency they are expressed in, and whether the series has been adjusted for corporate actions. A share that has split has a price history that reads one way unadjusted and another way adjusted, and a total return series including reinvested dividends sets records the price series never reaches.

The ambiguity is worse for constructed series. An index is a calculation with a base date and a divisor, so its record is a record of the calculation, not of any tradable thing. A continuous futures series is stitched together from successive expiries, and the stitching method, whether prices are back adjusted at each roll or simply joined, changes the history enough that two providers can disagree about whether a record has been set at all.

The record itself is a description, not a signal, and two conventions coexist that contradict one another. One treats a former high as resistance, on the reasoning that holders who bought lower are more willing to sell there. The other treats a market above its record as one with no overhead supply, because nobody holding it is waiting to break even. Both are taught widely and neither is established, which is a useful demonstration that a chart convention can be popular without being tested.