A validation rule is a tool that allows administrators to enforce data quality and integrity by preventing users from saving records that do not meet certain criteria or conditions. For example, a validation rule can display an error message if a user enters an invalid value in a field. In this case, the administrator can create a validation rule on the account object thatdisplays an error if the account annual revenue field is below 0 or greater than 100 billion.References:https://help.salesforce.com/s/articleView?id=sf.validation_rules.
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