∪ is the relationship between two sets, indicating the merger of two sets and the new set sorted according to the mutual difference of sets.

Sum is a result, and all the results calculated by+-can be called sum, where sum is a formula.

In addition, harmony can also express a coordinate relationship, which is from the perspective of Chinese.

Give a simple example. Set A and set B, A∪B, A and B are obviously different. The former is one set and the latter is two sets. Even though the elements are similar, the concepts are completely different.