A "unique subset" refers to a smaller group of items or elements that is distinct and different from any other group. It is a specific and separate part that stands out in some way. Full definition
In fact, the entire activity has its own unique subset of loot, complete with attributes designed to help defeat the activity in future runs. (coin-drop.com)
Each nursing specialty requires its own unique subset of skills, experience, and character traits. (resume-now.com)
So it's not surprising that something totally normal like listening to the radio is super uncomfortable for a unique subset of moms who just don't want to have their kids» ears exposed to cussing or sexual content. (babygaga.com)