Is the Thrift Savings Plan a 401k plan, a 403b plan, a 457 plan, or none of the above? If it is none of the above, then where are the rules and regulations of the plan defined if not in the U.S. Code?
It is none of those. It is defined in the US Code, but not the Internal Revenue portion of the code.
The official explanation:
The TSP is established under the Federal Employees' Retirement System Act of 1986 and is codified primarily under Chapter 84 of title 5, United States Code (U.S.C.).
The Internal Revenue section of the code is involved only insofar as to designate this type of plan as tax-exempt, according to the same source:
The TSP is treated as a qualified trust which is exempt from taxation (see 26 U.S.C. § 7701(j)).
For that matter, nearly all the rules and regulations for 401(k) plans are found in ERISA, not section 401(k).