Generally speaking, all definite and different objects in a certain range constitute a set (set for short).
Certainty For a given set, the elements in the set are certain. In other words, an element, whether it belongs to a set or not, must be one of them.
The disorder in a set, regardless of the order between elements, is considered to be the same set as long as the elements are exactly the same.
For a given set, the elements in the set are different from each other (distinguishable), and any two elements in the set are different. The same element and repeated elements, no matter how many, can only be counted as one element in the set.