The use of brackets on certain information on a form combines a grammar device with legal theory to legally isolate whatever is in the brackets from (thus, render it legally inapplicable and insubstantial to) the body of text within the surrounding contract.
The legal theory for this comes from the Four Corners Rule in law:
Under "four corners rule", intention of parties, especially that of grantor, is to be gathered from instrument as a whole and not from isolated parts thereof. Davis v. Andrews, Tex.Civ.App. 361 S.W.2d 419,423. (Black's Law Dictionary, 5th ed. p. 591) combined with the definition and description of square brackets found in the Plymouth University Foundation Degree guidelines on essay writing,
1. The term 'brackets' is commonly used to describe both square brackets [these] and round brackets (these) - whose technical name is ‘parentheses’
2. Square brackets...are used to indicate that something has been added to the original text for editorial purposes of clarification or comment.
The reporter added that this woman [Mrs. Wood] had suffered severe injuries.
A mother wrote that her son was 'frightened [sic] to go to school'.
3. Any statement within brackets should be grammatically independent of the sentence in which it occurs. That is, the sentence should be complete, even if the contents of the brackets were to be removed.
The republican senator (who was visiting London for a minor operation) also attended the degree ceremony.
The law states that contract meaning is derived from and only from within the "four corners" of the document (which is a "box"), but not from an isolated section of it. (Remember that in law, words and phrases are used precissionally, not generally. The legal phrase "not from isolated parts thereof" is a precisional statement. In other words, no meaning at all is to be derived from isolated contents within the "box" of the document.
Brackets inside of the four corners "box" create an implied "inner box" separated from the "outer box," grammatically and thus legally isolating the contents of the "inner box," rendering what is in the inner box as mere reference or comment but non-substantial to the outlying text of the contract in the outer box.
So when you bracket the [SSN] or the [zip code] or [U.S.] or anything else on a legal document, you are declaring that it is legally isolated, being merely grammatically referential to but not legally intrinsic to the contractual meaning of the contents of the surrounding document. According to the Four Corners Rule, no contractual meaning can be applied to or derived from any "isolated parts" within it.
In this way, one is offsetting a presumption of jurisdiction by virtue of inclusion of the SSN or zip code on the face of the document. The brackets are legally saying, "these inner contents are legally isolated from and not to be construed as part of the legal meaning of the surrounding contents. I am not identified by an SSN though it appears here. I am not a federal citizen though a zip code appears here. I am not a statutory corporate 'U.S. citizen'."