So this may be a really, really simple question, but I am having trouble understanding something. I was curious what a turqoise wire is, so I did some reading and found out it is for a refnum. But, I cannot find out what a refnum actually is. I think it is a link to a specific control on the front panel, but then why use a reference instead of a property node, or even wire the control directly to a desired output?
There are many uses for control references. For example you could feed the refnum of a control to a subVI and change some property of the control there (e.g. value, color, boolean text, etc.).
Here is some more information about the different applications of property nodes and refnums, in addition to the great info that altenbach posted. Hope that helps!