UC Index?

I recently found out about the UC Index and have filled out my stats :
3.875P GPA,
1470 SAT
750 R/W (35,40)
720 MATH
(this was my first time) taking it again in June to get 1500’s (hopefully)
and received an UC Index of 275 which places me in the top 9% of California Students.
I am just curious as to how this can help with admission as I plan to apply to UCSD, UCD, UCI, and UCSB.
And does this apply to Cal States?

http://admission.universityofcalifornia.edu/freshman/california-residents/admissions-index/

Expect the UC campus with space available to be Merced.

Most CSUs use a CSU eligibility index = GPA * 800 + SATR + SATM. This is 4570 for you. If you are a California resident, 2900 admits you to non-impacted majors at non-impacted campuses. However, most CSUs have some impaction, and some majors can be highly impacted. See http://www.calstate.edu/sas/impactioninfo.shtml .

Most CSUs do not publish their past admission thresholds, but SJSU does at http://www.sjsu.edu/admissions/impaction/ .

CPSLO uses a Multi Criteria Admission score described at http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1014&context=statsp rather than the usual CSU eligibility index.

An eligibility index for the CSU’s of 4570 makes you competitive for all. For Cal Poly SLO, you need to calculate your MCA points around 4700+ will make you competitive for the majority of the SLO majors.
CSU’s admit by major, so intended major can be a big difference on your chances especially with the impacted CSU campuses.

CSU transfer students that do not get into a preferred CSU, will have their applications forwarded to CSU campuses that still are accepting transfer students. Never heard that they do this for in-coming Freshman applicants in the CSU system, but as stated by @ucbalumnus, a non-impacted campus will accept you with a minimum EI.

i think the UCs you list are a stretch. If you just gotta go to a UC, apply directly to UCR and UCM, UCSC could go either way. Do not count on ELC admission to either. Note the language about space available - UCM may limit enrollment in the coming years. With your stats, you are in the running for most programs at every CSU except CP SLO - which would depend on your major. I’d encourage you to investigate and apply to SDSU, CSULB and other CSUs depending on your interests.

Freshman admit rates (2016) for UC GPA of 3.80-4.19:

UCB: 14%
UCLA: 14%
UCSD: 44%
UCSB: 54%
UCD: 58%
UCI: 65%
UCSC: 85%
UCR: 94%
UCM: 96%

Intended major is aerospace engineering, if that makes any difference.

Engineering majors are often more popular / competitive / selective than the campuses overall.

SLO for example has about a 15% acceptance rate for Aerospace Engineering, Average CP GPA of 4.17 and SAT of 1450 for the College of Engineering for 2016.

UCI had about a 35% acceptance rate in 2016 with an average UC GPA of 4.07. Should be interesting to see what the acceptance rate for this year will be since it continues to decline.

Engineering/STEM majors as stated by @ucbalumnus are usually the most competitive majors at all these schools.