A subset is a smaller group or part that is completely included within a larger group or set. It consists of some, but not all, of the elements or members of the larger group. Full definition
I think a flipped classroom is kind of subset of blended learning. (remakelearning.org)
Another very large share of statutory language creates legal obligations related to the regulation of very complicated activities that have application to a very small subset of people. (politics.stackexchange.com)
There is nothing in that comment about black people, and black people aren't the only subset of the population that commit crimes. (sbnation.com)