A boolean value stores information where only two possible outcomes can conceviably exist. In general, boolean values are either true (Yes) or false (No).
For example, the highest level of the hierarchy in each Crash Involved Vehicle type dimension has only two possible values. One of these indicates that the specified vehicle type was involved, the other indicates that it was not.
Boolean values in MAST are particularly useful in types of report which could potentially result in double counting.