What is an example of a polymorphic lookup field in Salesforce?

The `ParentId` field on the standard Account object is a self-lookup field, meaning it always looks up to another Account record, not multiple distinct object types.

The `LeadId` and `ContactId` fields on the standard Campaign Member object are two separate, distinct lookup fields, each pointing to a specific object type (Lead or Contact), rather than a single polymorphic field.

A custom field `Link__c` that explicitly looks up to *either* an Account *or* a separate custom object would typically be implemented as two separate lookup fields, or a master-detail if applicable, and would not natively achieve polymorphic behavior for a single field without significant custom development and data modeling that doesn't define it as a polymorphic field type.

The `WhatId` field on standard Event and Task objects is a polymorphic lookup, meaning it can relate to multiple types of objects, such as an Account, Opportunity, or any other object that can be associated with an activity, enabling flexible record relationships.