The Cascading Select custom field consists of two drop-down lists. Currently in the 'required' mode of this field, only the existence of a value in the first drop-down is validated. In some scenarios this may be not enough. There should be an additional configuration option for the field, such that administrators can define whether only the parent option, or both parent and child option must be set in 'required' mode.
An example support case requesting that functionality: https://support.atlassian.com/browse/JSP-58167.