Numerology's core-number system generally works by taking one input — a birth date or a birth name — and extracting a single figure meant to describe some facet of a person. The soul urge number is one of the more selective versions of this: rather than using an entire name, it isolates just the vowels (A, E, I, O, U, and sometimes Y depending on the school), converts them to numbers, and reduces the total.
That selectiveness is the whole premise. Numerology treats consonants as the part of a name that shapes how a person presents outwardly, and vowels as carrying something quieter and more internal — closer, the tradition claims, to what a person wants for themselves rather than what they show the world.
The result sits alongside two other core numbers derived from the same birth name: the expression (or destiny) number, calculated from every letter, and the personality number, calculated from the consonants alone. Soul urge is the vowel-only third of that trio.