The Federal Register Document Citation format recommended by the Office of the Federal Register is by volume and page number, regardless of whether you are using the printed edition or online editions.
The sidebar of Federal Register 2.0 articles contain the basic Federal Register citation; for example: 77 FR 12345. Additional page numbers appears along the right margin in the ribbon icons. Click on the ribbon to see the page cite.
Some legal journals require the inclusion of the date: 77 FR 12345 (Month Day, Year).
The Harvard Blue Book and ALWD style differs slightly, .e.g. 77 Fed. Reg. 12345 (Month Day, Year).