Everything else in this chart is built by addition. The challenges are built by subtraction, and they are the only positions that are. The Major Challenge is the absolute difference between the two minor challenges, which are themselves differences taken between reduced components of the birth date.
The choice of operation is doing the doctrinal work. Addition combines and produces something larger; the tradition uses it for what a person has. Subtraction measures a gap, and the tradition uses it for what is missing or unresolved. A challenge is described not as a quality but as a lack that has to be worked against, and the arithmetic is chosen to match.
Two consequences follow from taking a difference. The result can never exceed eight, so the challenge positions never yield a nine and never yield a master number; the highest possible gap between two single digits is eight, which caps the range in a way no other position is capped. And the result can be zero, which no additive position in the chart can produce.
Zero is given its own doctrine. It is read not as the absence of a challenge but as the hardest case, a challenge with no particular shape, sometimes described as the challenge of choice itself. That reading is not derivable from anything; it is a stipulation made to give the value a meaning, and it is the clearest instance in the chart of a doctrine written to cover an arithmetic outcome after the fact.
It occurs whenever the two minor challenges are equal.