Task work breakdown structure code is a read-only alphanumeric code that can indicate the relationships between tasks.
Service reference: http://ServerName:32843/[Project Service Application GUID]/PSI/Project.svc
Web service reference: http://ServerName/ProjectServerName/_vti_bin/PSI/Project.asmx?wsdl
The TASK_WBS is a read-only property that contains work breakdown structure (WBS) codes. WBS codes are alphanumeric values that you can use to represent the associated task's position within the hierarchical structure of the project. WBS codes are similar to outline numbers, except that you can edit WBS codes in Project Professional and you can apply a specific WBS format, or mask. WBS codes are unique; you can have only one WBS code per task. By default, Project Professional sets the WBS to the TASK_OUTLINE_NUM value (1, 1.1, 1.2, and so on).
Although the PSI and the Visual Studio Object Browser show it as read/write, the TASK_WBS property should be marked as read-only. The TASK_WBS property is the aggregate of the parent TASK_WBS value and the value of the TASK_WBS_RIGHTMOST_LEVEL field in the MSP_TASKS table. Because a TaskRow object does not include a property to set TASK_WBS_RIGHTMOST_LEVEL, you cannot programmatically set TASK_WBS.
If you try to set the TASK_WBS property to a specified value and then save the project, Project Professional ignores the value set with the PSI and assigns a value according to the task outline number when you open the project. To see the result in Project Professional, see the WBS code value on the Advanced tab of the Task Information dialog box.