A subset is a smaller group that sits completely inside a bigger group. Every member of the subset is also a member of the big group — that is the whole point. Cats are a subset of animals; every cat is an animal. The word comes out of mathematics, where it is exact: set A is a subset of set B if every single thing in A is also in B. Outside maths it keeps that shape but gets used more loosely, especially in business, science and computing: 'a small subset of our customers', 'a subset of the data', 'only a subset of the features work on the old phone'. In that everyday use it often carries a quiet extra meaning of 'not all of them, only some' — when a company says 'a subset of users were affected', part of what they are doing is telling you the number is small. It is a written, thinking-out-loud sort of word. In normal talk people say 'some of', 'part of', 'a certain group of'. Notice that a subset is not the same as a piece broken off. A slice of cake is a part of the cake but we would not call it a subset; subsets are about members of a group, things you could count and list. Also, strictly, a group counts as a subset of itself — maths allows that — though in real use people almost always mean a smaller-than-the-whole group.