The AssignmentRuleHeader must be specified in the create() or update() call of a Case or Lead for the specified assignment rule to be applied, and it must be specified in the update() call of an Account for the territory assignment rules to be applied.
The ID of a specific assignment rule to run for the Case or Lead. The assignment rule can be active or inactive. The ID can be retrieved by querying the AssignmentRule object. If specified, do not specify useDefaultRule. This element is ignored for accounts, because all territory assignment rules are applied.
If the value is not in correct ID format (15-character or 18-character Salesforce ID), the call fails and a MALFORMED_ID exception is returned.
If true for a Case or Lead, uses the default (active) assignment rule for a Case or Lead. If specified, do not specify an assignmentRuleId. If true for an Account, all territory assignment rules are applied, and if false, no territory assignment rules are applied.
For a code example, see Lead.