On the Standard Output/Error tab of the step settings for a Call Executable step, you can select an Output Destination for the standard output. The value you select here corresponds to an integer in Step.StdOutput.Dest: 0 is Ignore, 1 is Store in Variable/Property, etc. For Store in Variable/Property, you can enter an expression of where the data should be stored (for example, Locals.MyOutput). Two of the other options are to store the output in a file, either locally or on a remote machine. In these cases, a file browse control allows you to specify the file.
The expression you see in the Additional Results is designed to log the output information, and it must pull from any of these locations depending on which one the user selected. The first section in parenthesis basically says "If the user chose to store this output in a local or remote file, include the localized (translated into the current TestStand language) string for "Local File Value" or "Remote File Value"". The part at the end that says " + Evaluate(Step.StdOutput.Expr)" actually pulls the standard output data from wherever it was stored (local file, remote file, or a variable) and includes it in the string. The end result of this is that you get the standard output, and if you saved that output in a file, you get a tag in the string that lets you know the data is saved in a file.
I hope that helps, and let me know if you have any further questions about it.
