What is the difference between the two SAT score percentiles?

I just got my test score back and I came across two different percentiles on the score report - Nationally Representative Sample Percentile and SAT User Percentile National. It seems to me that the SAT User Percentile is only for students that are going to college?

If so, how does CollegeBoard know this and why is this something that is taken into consideration? Which is the one that is generally considered more important to colleges?

Yes, User Group Percentiles tell you how you compare against 11th and 12th grade students who take the SAT and plan on attending colleges (vs. Nationally Representative Percentiles, which are weighted to tell you how you compare against all 11th and 12th grade students). The only information currently available about how the College Board arrived at these percentiles is what the College Board itself tells you in your score report (bolding mine):

User group percentiles are derived via a research study sample of U.S. college bound students in 11th and 12th grades, weighted to represent students who typically take the SAT last as 11th or 12th graders. For example, a student’s score in the 75th percentile means that 75% of the user group of U.S. students last testing as 11th or 12th graders would have had scores at or below that student’s score.

Basically, the College Board arrived at the percentiles by giving the new SAT to a group of 11th and 12th grade students planning on attending college, seeing the distribution of the scores, and then mapping that distribution onto the scores of the officially administered exam. For instance, if in the initial group of students 50% of students got a 543 or below on Evidence-Based Reading and Writing, then if you got a 540 on the Evidence-Based Reading and Writing section when you took the SAT, you'd be in the 50th User Group Percentile.

The exact weighting that the College Board used is not currently publicly available (and probably won't be made publicly available, due to concerns about proprietary data and student privacy and so on), but that gives you the basic gist.

The reason the College Board gives you both the User Group Percentiles and the Nationally Representative Percentiles is that each percentile gives you different information. If they look at the percentiles at all, colleges will most likely care more about User Group Percentiles, since that's the group who will make up their applicant pool.