This document describes the current understanding of the IVOA controlled vocabulary for describing astronomical data quantities, called Unified Content Descriptors (UCDs). The present document defines a new standard (named UCD1+) improving the first generation of UCDs (hereafter UCD1). The basic idea is to adopt a new syntax and vocabulary requiring little effort for people to adapt softwares already using UCD1. This document also addresses the questions of maintenance and evolution of the UCD1+. Examples of use cases within the VO, and tools for using UCD1+ are also described.
@article{arxiv.1110.0525,
title = {IVOA Recommendation: An IVOA Standard for Unified Content Descriptors Version 1.1},
author = {Sébastien Derriere and Norman Gray and Robert Mann and Andrea Preite Martinez and Jonathan McDowell and Thomas Mc Glynn and François Ochsenbein and Pedro Osuna and Guy Rixon and Roy Williams},
journal= {arXiv preprint arXiv:1110.0525},
year = {2019}
}