Referencing a standard object in Custom Object Rules in BUI

Content

Hi, 

We've just had a test site upgraded to 21C and I'm trying to figure out how I can use custom object rules to set an incident or contact field when a field on the related custom object is changed. I'm assuming this is possible based on https://documentation.custhelp.com/euf/assets/devdocs/buiadmin/topicrefs/t_bui_Edit_biz_rules_custom_objects.html which says: 

Note: You can create rules that reference objects related to the custom object for which the rule is created. As you create rules, their details display in the Summary section. Clicking a condition or action in the Summary section will highlight it there, as well as in the condition or action section.