C.F.R. is an abbreviation for the Code of Federal Regulations. Whereas the U.S.C. contains the federal statutes promulgated by Congress, the C.F.R. contains the federal regulations promulgated by the Patent and Trademark Office that provide the details omitted from the statutes.
For example, 35 U.S.C. 121 broadly authorizes the Commissioner of the PTO to require a patent applicant to restrict a patent application to a single invention if a patent application is filed that discloses two or more distinct inventions. 37 C.F.R. sections 1.141, 1.142, 1.144, and 1.146 provide details that implement the broad requirements of the statute.
There is no numerical correspondence between the U.S.C. and the C.F.R. However, there is a clear numerical correspondence between the C.F.R. and the Rules of the PTO. For example, 37 C.F.R. 1.131 is the same as Rule 131 of the PTO.