When a notice sent by the Office to any registrant or applicant is returned to the Office undelivered, including notification to the Office of non-delivery in paper or electronic form, additional notice may be given by publication in the Official Gazette.
37 C.F.R. §2.118
Part 3 pertaining to both patents and trademarks is placed in the grouping pertaining to patents regulations.
Part 4 is placed in the separate grouping of parts pertaining to patents regulations.
Part 5 is placed in the separate grouping of parts pertaining to patents regulations.