The information is hidden away in Standards of Chess Equipment and tournament venue for FIDE Tournaments section 13.4.
13.4. Buchholz System
13.4.1. The Buchholz System is the sum of the scores of each of the opponents of a player.
13.4.2. The Median Buchholz is the Buchholz reduced by the highest and the lowest scores of the opponents.
13.4.3. The ...