The best way to understand the concept of a subset is to think about a canary deployment, where you split traffic into two subsets based upon labels such as v1 and v2 and then gradually shift the traffic to make a canary deployment production. Finally, you remove the old production deployment from service.

The term canary comes from the practice of taking a caged canary (bird) into a mine where the bird may die from carbon monoxide poisoning to ensure the miners don't die. This is because carbon monoxide is odorless. In software, the same term is used to send a small portion of the traffic to a newer service and expose it to a small set of friends, family, trusted users, and so on to gauge its worthiness. Let's think of another example. Let's say that you want to expose a brand new UI that works only on iPhone or Android to a select group of people to gather feedback. You can expose the new service with limited traffic flow to users that match the request routing (headers and so on).