Custom validations allow administrators to enforce business rules on transactions beyond standard system requirements by evaluating condition rules and displaying a message when conditions are or are not met. The Official Workday Pro Procure-to-Pay Guide explains that custom validations can be configured to display either a Warning message, which alerts the user but allows them to continue, or an Error message, which blocks the user from proceeding until the issue is corrected. This dual capability gives administrators flexibility to either guide users with soft reminders or enforce hard stops for critical business rules. Option A is incorrect because restricting custom validations to warnings only would prevent administrators from enforcing mandatory rules that must block submission. Option C is incorrect because limiting to errors only would remove the ability to provide non-blocking guidance, which is a commonly used and valuable feature. Option D is incorrect because the entire purpose of a custom validation is to communicate a message to the user when a defined condition is triggered; a validation that displays no message would serve no functional purpose.