The article from Cornell linked covers definitions used within the "U.S. Code 28 Part VI (PARTICULAR PROCEEDINGS) Chapter 174 (FEDERAL DEBT COLLECTION PROCEDURE)".

Federal debt collection, and who is the recipient of the debt collected?

Federal meaning central government, so why would the definition for "United States" in terms of federal debt collection refer to "United States" as a corporation as one of the options in terms of defining what "United States" means in the context of collecting federal debt?

Try to combine the sentences into three different sentences with the given prefix:

"United States" means a Federal corporation;

"United States" means an agency, department, commission, board or other entity of the United States.

"United States" means an instrumentatlity of the United States.

How are you able to discern that (B) related to things owing debt? Read the second sentence constructed above combining the prefix and (B).

Those three definitions defined what "United States" is defined as in terms of who the recipient of federally collected debt can be when debt is collected.

(C) means a subsidiary branch of the central government or an organization that works for the central government.

EDIT: In the case of point (A) I will submit to the fact that it can also mean a federal corporation incorporated through congress.

Thus the question becomes, who can incorporate a new corporation? Interesting reading: "Corporate personhood"