Validity criteria help page [1] does not provide a description/samples for "Custom" selected from "Allow" list. See [2], [3] for questions about that. [1] https://help.libreoffice.org/6.4/en-US/text/scalc/01/12120100.html [2] https://ask.libreoffice.org/en/question/229812 [3] https://ask.libreoffice.org/en/question/225447
Moving to NEW
Seth Chaiklin committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/9512494c315aee4af46fa9a9eca0e4be0210e3cd tdf#138301,tdf#130738 add "Custom" condition in Data>Validity-Criteria help