So a little background will help. the United States Code has only been around since 1926 where before the law was a patchwork of acts. Over time congress has taken a bundle of acts and relegated them to be covered by a title in the code. This project is still on going with legislation for Title 52 and Title 54 passing in 2014.
Title 53 is a work in progress intended at this time for Small Business matters but has not yet had legislation approved for it so at this time the legislation is reserved. Here is an explanation from Congress over H.R. 6389 (115th Congress):
Originally enacted as title II of the Act of July 30, 1953 (ch. 282, 67 Stat. 232), the Small Business Act of 1953 was made a separate Act, renamed the Small Business Act, and revised by Public Law 85–536 on July 18, 1958. A month later, by Public Law 85–699, Congress enacted the Small Business Investment Act of 1958. Since 1958, the Acts have been amended a number of times and have been supplemented by provisions in other Acts that, while they do not amend either Act by their terms, have the effect of modifying the Acts. Some of the amendments added provisions that are now obsolete, and some added provisions that are inconsistent with other provisions.
The Acts are classified to separate chapters of title 15 of the United States Code. The bill restates the Small Business Act (15 U.S.C. 631 et seq.), the Small Business Investment Act of 1958 (15 U.S.C. 661 et seq.), and related provisions of other Acts as a new positive law title of the United States Code. The new positive law title replaces the former provisions, which are repealed by the bill.
The bill was prepared by the Office of the Law Revision Counsel as part of the Office’s ongoing responsibility under section 205(c) of House Resolution No. 988, 93d Congress, as enacted into law by Public Law 93–554 (2 U.S.C. 285b), ‘‘[t]o prepare, and submit to the Committee on the Judiciary one title at a time, a complete compilation, restatement, and revision of the general and permanent laws of the United States’’.
The bill is a codification measure prepared in accordance with section 205(c) of House Resolution No. 988, 93d Congress as en- acted into law by Public Law 93–554 (2 U.S.C. 285b). The purpose of the bill is to enact a restatement of certain existing law relating to small business as a positive law title of the United States Code. The restatement of existing law does not change the meaning or effect of the existing law. The restatement consolidates various provi- sions that were enacted separately over a period of many years, reorganizing them, conforming style and terminology, modernizing obsolete language, and correcting drafting errors. These changes serve to remove ambiguities, contradictions, and other imperfections, but they do not change the meaning or effect of the existing law or impair the precedential value of earlier judicial decisions or other interpretations.