WhatId vs WhoId in Salesforce: What They Mean & When to Use Each

WhatId is a polymorphic relationship field on the Task and Event objects that points to a non-person record — Account, Opportunity, Campaign, Contract, or any Custom Object that has activities enabled. It's the 'Related To' field in the Task/Event UI. WhatId is required when the task isn't linked to a person.

WhoId references people — specifically a Lead or a Contact. WhatId references things — Account, Opportunity, Campaign, custom objects, etc. Both are polymorphic lookup fields that exist only on Task and Event records. A given activity can have one of each, but only one Who and one What at a time (unless Shared Activities is enabled, which allows up to 50 Contacts on a single activity).

No. WhoId only accepts Lead or Contact IDs. If you assign an Account ID to WhoId in Apex, the DML throws an INVALID_FIELD_FOR_INSERT_UPDATE error. To attach a Task to an Account, use WhatId instead. The Task's UI 'Name' field maps to WhoId, while 'Related To' maps to WhatId.

Yes — both are polymorphic lookup fields. You query the related object via TYPEOF in SOQL: SELECT Id, WhatId, TYPEOF What WHEN Account THEN Name END FROM Task. In Apex, cast the result based on Schema.SObjectType: String objType = task.WhatId.getSObjectType().getDescribe().getName();

Out of the box: Account, Opportunity, Campaign, Contract, Order, Solution, Asset, Quote, and Product2. Any Custom Object becomes a valid WhatId target once you enable 'Allow Activities' on the object definition. Cases and Leads are intentionally excluded — they have their own activity relationships.

Use the relationship name 'What' (without Id) and TYPEOF in SOQL: SELECT Id, Subject, TYPEOF What WHEN Account THEN Name, Industry WHEN Opportunity THEN StageName END FROM Task WHERE Id = :taskId. The TYPEOF clause is required because What is polymorphic — Salesforce cannot guess the field set otherwise.

Yes, but with two caveats: (1) If Shared Activities is enabled, you should use the TaskWhoRelation/EventWhoRelation junction objects to add multiple Contacts instead of overwriting WhoId. (2) Updating either field re-fires assignment rules and triggers — be careful in bulk operations to avoid governor-limit issues.