Specific task collections are located within a specific work item. You must first navigate to the specific work item and then to the specific collection. For navigating to a specific item you need to execute one of the following URLs entering the relevant work item ID:
From the Workflow Generic Task service:
Only the collections that were defined in the Generic Task service are displayed.
http://<machine name>:<port>/sap/opu/sdata/sap/wfgentaskrocessing/WorkflowTaskCollection(value=<workitem_id>,scheme_id=IW_WF_GEN_TASK,scheme_agency_id=<SAP_system>_<client>)?$format=xml
From the Workflow User Decision Task service:
Relevant for Workflow items of type User Decision.
http://<machine name>:<port>/sap/opu/sdata/sap/wfdecisiontaskrocessing/WorkflowTaskCollection(value=<workitem_id>,scheme_id=IWF_USER_DECISION_TASK_GW,scheme_agency_id=<SID>_<client>)?$format=xml
From the Workflow Dialog Activity Task service:
Relevant for Workflow items of type Dialog Activity.
http://<machine name>:<port>/sap/opu/sdata/sap/wfactivitytaskrocessing/WorkflowTaskCollection(value=<workitem_id>,scheme_id=IWF_ACTIVITY_TASK_GW,scheme_agency_id=<SID>_<client>)?$format=xml
Once inside the work item, you can reach the specific collection.