For variables data, a unit either meets specifications or does not meet specifications. Consequently, a sampled unit is either defective or not defective.

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

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