What is the difference between  WORK_QUEUE_POLICY_ID and DISPATCH_WORKQUEUE_POLICY_ID?

The DISPATCH_WORKQUEUE_POLICY_ID is used to create a POA with a work queue policy other than the default one.

The old policy name WORK_QUEUE_POLICY_ID has changed as is covered in the migration guide:

Work queue policy ID The policy ID that identifies the manual work queue policy has changed in Orbix 6.3. That is, the IT_WorkQueue::WORK_QUEUE_POLICY_ID policy ID has changed to IT_PortableServer::DISPATCH_WORKQUEUE_POLICY_ID.
For example, the ASP 5.1 code for creating a manual work queue policy on the POA would include the following line:
Whereas the Orbix 6.3 code for creating a manual work queue policy would
include a line like the following:
// C - ASP 5.1
...
policies[0] = global_orb->create_policy(
IT_WorkQueue::WORK_QUEUE_POLICY_ID,
workQueuePolicy);
...
// C - Orbix 6.3
...
policies[0] = global_orb->create_policy(
IT_PortableServer::DISPATCH_WORKQUEUE_POLICY_ID,
workQueuePolicy);
...

References to Written Documentation:

Migrating from ASP 5.1 to Orbix 6.3,
Chapter 2 - Source Code Modifications