Attachment of the ion
\(\ce{H^{+}}\) either in natural abundance or where it is not desired to distinguish between the isotopes.
Source:
PAC, 2022, 94, 353. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)' on page 435 (https://doi.org/10.1515/pac-2018-1010)