Includes those components of the index which comprise the top or bottom x percent of capitalization, as you define. To pick the appropriate stocks, we sort the list by capitalization (Top sorts the list descending from largest cap to smallest cap) and get the sum total capitalization of all components. Then we include stocks from the top of the list until the percent of capitalization reaches the percent you set.

For example, suppose you choose Select the components which comprise the TOP 50% of capitalization of our imaginary index ZYX@IBX. This index is composed of 10 stocks whose caps range from 10 to 100 in increments of 10, like this: A=10, B=20, C=30, D=40, E=50, F=60, G=70, H=80, I=90, J=100. The sum total capitalization is 550. To get your 50% of the top caps we need 50% of 550, or 275. We start at the top and take: J, I, H (which totals 270). We stop here, since this is the closest we can get to the 275 goal.