The Software Package Data Exchange (SPDX) says that it is an "open standard for communicating software bill of material information, including provenance, license, security, and other related information".
The SPDX license list includes standard names for some complicated public-domain dedications, such as CC0-1.0. Unfortunately, the license list does not currently include standard names for simple public-domain dedications (such as the simple text "Public domain"), even when those dedications are commonly used.
Fortunately, SPDX is user-extensible. This page defines the following names for the following simple public-domain dedications:
All of these dedications are already commonly used, modulo substitutions of specific names for "this work". Perhaps these will be added to the standard SPDX license list as PD-2, PD-d, PD-hd, PD-hp respectively, following SPDX's decision to "treat each unique 'Public Domain dedication' in the same way as any license is treated in the standard". In any case, for continuity, this page will continue to define LicenseRef-PD-2 etc.
For comparison, LicenseRef-Fedora-Public-Domain is a generic name for many different texts, contrary to Annex B of the SPDX specification.
Related reading: See "Placing work into the public domain" for information (including specific quotes from court decisions) on the legal effect of simple public-domain dedications.