Check the NI Web site for "How do I Access the UUT Serial Number in My Sequence or Test Code?" at ..... RESOURCE LIBRARY>TESTSTAND>SEQUENCE EXECUTION>GENERAL
It states ...
At runtime, you can access the UUT serial number using Runstate.Root.Locals.UUT.SerialNumber. The serial number and UUTLoopIndex is a local variable of the default process model. Below is the definition for Runstate.Root. This is copied from Chapter 8 of the TestStand User Manual.
Runstate.Root: Sequence context for the root sequence invocation. If you initiate an execution using a process model entry point, the property is the sequence context for the process model entry point. For example, if you use an entry point from the default TestStand process model, the Root property is the sequence context of the Test UUTs or the Single Pass sequence. If you initiate an execution on a sequence without using a process model entry point, the Root property object is the sequence context for the sequence you run.
When you are editing a sequence, the expression browser does not display the sub-properties of Runstate.Root because these properties are filled in at runtime. They are dynamic properties.