When the owner is deceased or legally incapacitated
Primary source
Verbatim text below is from the Electronic Code of Federal Regulations (eCFR), a public-domain U.S. government work. Always verify the current version with the eCFR before relying on it for any legal matter.
Full Text
In case of the death of the owner or if the owner is legally incapacitated, the legal representative (executor, administrator, or guardian) or heir or assignee of the deceased owner may sign as the applicant. If an applicant dies between the filing of his or her application and the granting of a certificate thereon, the certificate may be issued to the legal representative, heir, or assignee, upon proper intervention.
This is legal information, not legal advice. Laws vary by jurisdiction and change frequently. Always verify current law with official sources and consult a licensed attorney in your jurisdiction for advice on your specific situation.