Notes
A lookup filter in Salesforce is used to limit the records that are available for selection in a lookup field. It can be configured on the lookup field to ensure only specific records meet certain criteria, such as filtering out inactive accounts or only showing contacts related to a particular account. For example, if you're creating a custom field for selecting a related contact on an Opportunity, a lookup filter could be set to only allow contacts that belong to the account associated with that Opportunity.
Unlike a validation rule, which checks field values on a record after it is saved, a lookup filter limits the options available in the lookup field before a record is saved. This ensures that users select a record that meets the specified criteria, preventing errors during data entry.