Digital Root
Digit sum, summed down to a single digit.
Sum the digits, then sum the digits of the result, until one digit remains: 763 → 7+6+3=16 → 1+6=7, so 7 is the digital root of 763. The digital root of 0 is defined as 0; every other number lands between 1 and 9.
You never actually have to iterate: the digital root has a mod-9 shortcut — every number is congruent to its digit sum modulo 9, so the digital root is just n mod 9 (taking 9 when the remainder is 0, and 0 for n=0). This powers the ancient checking trick "casting out nines": take digital roots of both sides of an addition or multiplication and they must match, or the arithmetic is certainly wrong.
The distribution is almost perfectly flat: across 0–1,000,000, digital root 1 has 111,112 holders (including 1,000,000 itself), roots 2–9 have 111,111 each — about 11.1% apiece. The lone exception is root 0, held only by the number 0 itself: a theoretical singleton.
In RNGdle, the digital root appears in the Digit Profile for display only and is not scored: it is congruent to the digit sum (mod 9), so the digit-sum family already covers everything it knows — scoring it too would hand every number duplicate EP. For the scored version, see the Digit Sum entry.