Hey,
It seems that you have a few different features that you want to implement in your Operator Interface. Here is a description of how you could the existing Operator Interfaces could perform these tasks.
1. User Login: Both the Simple and Full Featured Operator Interfaces (which can be found in the following directory <TestStand Directory>/OperatorInterfaces/NI) will pop up a user login screen when the application is opened
2. Choose Particular Sequences Based on UUTs: Implementing a window that pops with a list of UUTs would involve customizing the Operator Interface. On the other hand, you could use the Full Featured Operator Interface and load all of the Sequence Files the user might want to run. If you name these sequence files according to the UUT IDs, the would be able to select the required Sequence File
3. Selecting Particular Tests: Both Operator Interfaces give you the ability to select particular steps and run them using the Single Pass entry point. (To perform this task, select particular tests and then right click and select [Run selected steps using >> Single Pass]) This is more of a debugging feature, but I believe it is what you are looking for. Furthermore, it doesn't involve any customizing of the Operator Interface.
Using the previous suggestions will not involve any coding or editing of the process model or Operator Interface. If these suggestions are not adequate a good place to start would be the customization of Operator Interfaces web event which you can go to by clicking on the following link:
Developing a Custom NI TestStand Operator Interface in LabVIEW
[http://sine.ni.com/apps/we/nievn.ni?action=display_offerings_by_event&event_id=18146&event_subtype=WEB_EVENT_DEMAND&site=NIC]
Regards,
Santiago D