Contract size

The quantity of the underlying that one contract covers. In foreign exchange one convention covers the whole class, a standard lot being a fixed number of units of the base currency. Commodities have no single convention, because the unit is physical: gold per troy ounce, crude oil per barrel. One lot is a different exposure in two commodity markets, and the specification states what that exposure is.

Everything downstream is built on it. Notional value is the number of lots multiplied by the contract size and then by the price, and both pip value and tick value are the contract size multiplied by the smallest increment the instrument moves in. Change the contract size and every one of those figures changes with it, which is why the specification, rather than a platform's default volume, is the thing worth reading before a size is chosen.

The trip is assuming a lot means one thing. Mini and micro lots are stated fractions of the standard lot, index contracts are commonly written as a money amount per index point rather than as units of anything, and share contracts are written on a number of shares. Two instruments quoted at similar prices can therefore carry very different exposure for the same stated volume.