Hi,
In LabVIEW we have "controls" (inputs) and "indicators" (outputs). To pass values into a sub-vi the value is passed into one of its controls, and to get values out of a sub-vi they come out of the indicators. Of course, this is all ignore control references, property nodes, etc. that allow you to "cheat". So when your main vi writes some text into the sub-vi it is to a string control. You can make this control "off screen" or invisible, and then wire it to an indicator, but it isn't necessary. Now, if you have buttons to signal the operator's response to the message sent in they would be wired to indicators, which also may be "off screen" or invisible, which would be wired to the connector pane. In the standard usage the inputs (controls) are wired to the left hand of the connector pane, the outputs (indicators) to the right hand ones.
If this isn't what you need, keep asking and we'll keep answering!
P.M.
PutnamCertified LabVIEW Developer
Senior Test Engineer North Shore Technology, Inc.
Currently using LV 2012-LabVIEW 2018, RT8.5
LabVIEW Champion