Validation error location and multiple-rule display
Key Takeaway
Show validation errors on a field whenever possible. When several rules fail on one save, Salesforce displays all of their messages at once.
Every validation rule has an error message and an Error Location, which controls where that message appears when the rule blocks a save. When several rules fail on the same save, Salesforce shows all of their messages together, so the way you split rules and place each message shapes what the user sees.
What happens when a rule blocks a save
A validation rule blocks a save when its formula returns True. Salesforce then shows the error message you wrote and does not save the record. Like other error messages, the message appears in red text and begins with the word "Error".