The phrase "certain subset" refers to a specific group of items or individuals that have common characteristics or qualities within a larger group. It is a smaller portion or division of something bigger. Full definition
The next step is to determine if phenformin alone would be a sufficient therapy for certain subsets of NSCLC or if the drug would perform better in combination with existing cancer drugs. (salk.edu)
As the report clarifies: «This is not to say that only those [employee] segments deserve attention, but rather that certain subsets of your organization require extra attention due to their importance to your organization's ability to innovate and grow.» (entrepreneur.com)
You have a certain subset of people who have wielded power and influence [for a long time]. (entrepreneur.com)