Let's say we have 100 sections, and for each section we can find how many sections that are smaller than it. For example:

Section 1: there are 20 sections that are smaller than section 1

Section 2: there are 95 sections that are smaller than section 2

Section 3: there are 30 sections that are smaller than section 3

...

And then from these sections, you pick out the ones that have 90 or more sections (i.e. more than 90% of the sections) that are smaller than them, and that's your answer.