Create a validation rule that uses the PRIORVALUE function is how an app builder can meet the requirement of preventing users from increasing the value of Estimated Hours after it is set. According to the Salesforce documentation, “PRIORVALUE returns the previous value of a field.” The validation rule can compare the current value and the prior value of Estimated Hours and display an error message if the current value is greater than the prior value. Creating a formula default value for the custom field, creating a formula field that uses the PREVGROUPVAL function, and creating a validation rule that uses the ISCHANGED function are not valid or correct solutions for this requirement.