Here’s more detail about the ELC (“local context”) determination:
http://admission.universityofcalifornia.edu/freshman/california-residents/local-path/index.html
I think the important thing to understand is that it is a statistical determination tied to a specific set of grades, in particular course, not something tied to actual class rank. So it is very possible that the “top 9%” could include students who have an in-school class rank that is lower than that, or exclude some students who are ranked higher. That’s partly because only UC-approved coursework from 10th & 11th grade is included in the calculation - while an individual student’s GPA and high school rank at the end of 11th grade would potentially include more classes and grades from 9th grade as well.
So it might help for you to think of it as a qualification that your son meets if his GPA is at a level that “on average” has historically been top 9%.
The statewide context top 9% – described here - http://admission.universityofcalifornia.edu/freshman/california-residents/admissions-index/index.html – is also factoring in test scores, so it can get pretty far afield from class rank. A B student with high scores might come out ahead of an A student with weaker scores. So again, it really is a calculation based on whether the students Academic Index falls in the top 9% based on historical data.
You can see at http://admission.universityofcalifornia.edu/freshman/california-residents/admissions-index/instructions.html (section 4) that these academic index numbers have already been determined – it is not something that would be directly impacted by the actual grades and test scores of the 2019 applicant pool. So really, in that context the “top 9%” part is something that is important to the state in predicting and controlling enrollment – for applicants it’s easier to just focus on the specific cutoff numbers.
I’d also add that historically there was a time that the ELC (local) status conferred a very significant admission advantage over statewide status – but I don’t think that it carries the same weight these days. At the time that it was more advantageous, it was limited to top 4% at the high school, and there were some campuses that essentially guaranteed admission to those students.