An optional value in a dataset is a field that can quite reasonably have no meaningful value, for example, the heart rate in a medical record if no measurement was taken. This is related to a missing value, which is when a field that is expected to have value has none, for example if the hourly reading from a sensor failed. Note that in both cases the absence of the value may itself carry information (e.g. that a test was not requred) and is different from a zero value.
Used in Chap. 10: page 148