Of the five Human Design types, the Reflector is the only one defined by nothing being present rather than by something being present, and that makes it structurally unlike the other four.
The rule
Human Design calculates a chart from a birth time and marks each of the nine centres as defined or open. The other four types are distinguished by which centres are defined and how they connect. The Reflector is the case in which none of the nine is defined.
Because it is the empty case, it is the rarest by construction rather than by assertion: it requires every one of nine independent conditions to fail at once, which happens for a small fraction of birth times. That rarity is arithmetic and can be computed by anyone with the calculation and a set of birth data.
What follows and what does not
The frequency follows from the rule. The system's account of how a person of this type functions does not follow from the rule; it is an interpretation attached to the empty case, and no independent published test establishes it.
Provenance
The typology is Human Design's own. Neither the I Ching, which supplies the sixty-four gates, nor the chakra model, which supplies a nine-centre adaptation of a classically sevenfold scheme, nor astrology, nor the Tree of Life, contains it. It was introduced by Ra Uru Hu in 1987 and the atlas records it as such.