What is the Constitution referring to when it states that "actual Enumeration shall be made within three years after the first Meeting of the Congress (1787) of the United States, and within every subsequent Term of ten years"?

The constitution was referring that:

To ensure that states were represented in proportion to their population, there is an establishment that every 10 years, every adult in the country must answer a survey. It is a monumental task because there is movement of people as often as they do and when some people have no homes at all.

On the basis of these surveys, Congress must determine how many representatives are to come from each state. At least one representative is required from each state. Also, it determines how federal resources are to be distributed among the states.

It also provided that each state shall have at least one U.S. House member.

The Constitution set the number of House members from each of the original 13 states that was used until the first census was completed.

This enumeration is what is now known as the U.S Census.

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