For variables data, you either say that a unit meets specification or does not meet specification. So a sampled unit is either defective or not defective.

You need to specify at least one specification limit, upper or lower, but many times you will specify both limits because a unit can usually be considered defective when it exceeds either specification limit (i.e it can be either too big or too small).

For the camera lenses, you specify both an upper (0.415) and a lower specification (0.395) because a camera lens will be defective if it is either too thick or too thin.