You can restrict the users to which a task can be reassigned or routed by using a callback class.

The user community seeded in a typical LDAP directory can represent the whole company or division. However, it may be necessary at times to limit the potential list of users to associate with a task based on the scope or importance of the task or associated data. For example, in a large company with thousands of users, only a few people have the ability to approve and create purchase orders. Specifically for such tasks, the users that can be chosen for ad hoc routing and reassignment should not be the whole company. Instead, only a few users who are relevant or have the right privilege should be chosen. This can be achieved by the restricted assignment functionality. This is implemented as a callback class that can implement the logic to choose the right set of users dynamically based on the task object that is passed containing the instance data.