I don’t think that “level of Interest” is a continuum on which there are different stopping points. Rather, the college either considers it or does not consider it, and had to choose a box to check on the CDS.
As for how to demonstrate interest, there have been many threads own the topic. Here’s one:
http://talk.qa.collegeconfidential.com/college-admissions/1626043-ways-to-show-a-high-level-of-applicants-interest.html