The molecules not used to devise a computational model but instead are used during model development to test for possible over-fitting as seen when additional
properties are added to a model but the predicted potencies of the validation set become less accurate.
Source:
PAC, 2016, 88, 239. 'Glossary of terms used in computational drug design, part II (IUPAC Recommendations 2015)' on page 259 (https://doi.org/10.1515/pac-2012-1204)