Briefly...and I would suggest looking these up for accuracy
UCLA has three programs, each one somewhat independent from the other
1. Charles Drew/UCLA Program
-separate AMCAS application, secondary, interview, etc.
-those students that the program wants to accept (approx. 12 students) have to go through a final review/screen by UCLA's admiss. comm.
-students complete 1st two years in westwood (with the rest of the class) and then complete their 3rd/4th yr at a county hospital. Rotations used to take place at MLK, but I believe they are branching out even more to Harbor-UCLA and Olive View county hospitals.
-as part of their 3rd/4th year, students are also req'd to propose/carry-out a research project
2. UCLA Geffen (aka. "regular" program)
-has it's own AMCAS, secondary, interview
-as part of the admissions process, UCLA has a variety of sub-committees, including one that reviews applications from "disadvantaged" applicants.
3. UCLA PRIME - this is the most recent addition, first entering class this year
-I believe the plan is for each UC medical school to have a PRIME program, though each will have a different focus. For example, UC Davis' program focuses more on rural medicine, whereas I believe UCI's focuses more on the Latino community. I think for UCLA, the PRIME program offers students a chance to work extensively with medically underserved communities in the LA area.
-it's a 5-yr program where students also earn their MPH (pls. double check this)
Regarding the disadvantaged section, it gives students an opportunity to explain their situation in more detail...things that might have impacted their education/learning during their formative years (this being the key time frame). Keep in mind that there is economic and also educationally disadvantaged.
Identifying yourself as disadvantaged will not "get you in"... it will simply offer the admission's committee a greater insight. Though I wouldn't go looking for things to put here just for the sake of it...it could work against you.
While this is a subjective section, my experience is that schools approach it very objectively. As WammaBamma stated, they (and other schools as well) ask for a lot of facts and figures. Members of the admissions committee have been doing this for a long time I'm sure, and know how to interpret the data and put in the context of your overall application.
Hope this helps, and if I mis-stated something, please feel free to edit.