The term is used several times at https://sfgov.org/civilservice/rule-111a-position-based-testing and I can't fathom the meaning. I searched the site and found a definition (copy below, indented).
But even after (roughly) understanding what eligibles, appointing officer, and more are, I still can't grok it. To me it seems akin to intentionally obfuscated source code. I can't grok what's in italics. Even after finding this help while drafting this. I can't grok what it means to consider a score. I think "employee organization" = union, but I'm not sure.
Sec. 202.6 Certification - Rule of Three Scores
The names of eligibles with the three highest scores on the list of eligibles for a position, who are available for appointment, are certified to the appointing officer. When there are two or more approved personnel requisitions on file for the same class, the number of scores certified shall be equal to the number of positions to be filled plus two scores. An eligible list adopted under the Rule of Three Scores shall in all cases be exhausted when eligibles standing at less than three scores are available. Use of the eligible list when there is less than the minimum certification available shall be at the discretion of the appointing officer.
If all eligibles at a score waive appointment or fail to respond within the time limits provided in these Rules, the appointing officer may request supplementary certification(s) from the next highest score.
Is it language designed to be impenetrable other than to a union's lawyers? That's my hunch, but it's just a WAG I'm hesitant to even mention.