A subset can include any related items, not just numbers.
Painters such as Monet and Degas are a subset of Impressionist artists.
A proper subset doesn't contain all elements of the original set. The set {1, 2} is a proper subset of {1, 2, 3}.
A set with all items from another set is a subset but not vice versa.
The set of all fruits is a subset of all foods, but they are not the same.