Where the Coast Guard issues numbers, a tender is exempt from separate numbering if it has propulsion machinery of less than 10 horsepower, belongs to the owner of a numbered boat, is used only to carry people between that boat and the shore, and displays the mother boat's number followed by the suffix "1" (33 CFR 173.13(b)).
The suffix is spaced like the other groups: a space or hyphen as wide as a letter or numeral, for example DC 5678 EF 1 or DC-5678-EF-1 (33 CFR 173.27(d)).
Check with your state
In the states listed in Appendix A, the state is the issuing authority, and its own rules decide whether a tender needs its own number. Read your state's rule before you letter a dinghy with the suffix.