What Destiny / Expression measures
Mulank comes from the birthday. Destiny comes from letters. The Expression number is the Chaldean total of the spelling you actually use, reduced to 1–9. Practitioners treat it as the “public job description” of a name — an invitation the spelling repeats, not a court order.
Drop an H, add a surname, or switch to a nickname and the total moves. That is why the form asks for the living name rather than a document you never speak.