Cross-object fields in validation rules
Key Takeaway
A validation rule can check a field on a related record, using relationship names joined by periods. The rule blocks the save when its formula returns True.
A cross-object reference lets a validation rule check a field on a related record, not just on the record being saved, so a rule can block a save based on data stored on a parent or lookup record. For example, a rule on a case can check a checkbox on the case's account.
How a validation rule blocks a save
A validation rule checks the data a user enters before the record can be saved. Its formula returns True or False. When the formula returns True, the rule blocks the save and the error message appears, so write the formula to describe the invalid condition.