I just came across a salesforce documentation to check the limitation of validation rules - https://help.salesforce.com/apex/HTViewHelpDoc?id=fields_validation_considerations.htm&language=th
It says:
Validation Rule Field Restrictions:
Validation rule formulas don’t or can’t refer to:
Compound fields, including addresses, first and last names, and dependent picklists and lookups
Campaign statistic fields, including statistics for individual campaigns and campaign hierarchies
Merge fields for auto-number or compound address fields such as Mailing Address
I tried to refer the lookup field in the validation rule of an object and I was able to validate whether the lookup is blank or not and show an error if the lookup field is blank. I don't understand what it means by
Validation rule formulas don’t or can’t refer to Compound fields, including addresses, first and last names, and dependent picklists and lookups
from the documentation link.