Given a log line from Apache such as the following:
xx.xx.xx.xx - - [04/Jan/2016:10:42:27 -0800] "GET / HTTP/1.1" 200 6861 "-""Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.106 Safari/537.36"
What is the cause of the "-" in the referrer location? I'm guessing that means the information wasn't available, but I'm trying to understand what causes this. Does it mean that this visitor typed the URL directly in the location field of their browser? Does it indicate a bot rather than a human? Or maybe the question should be what are the different actions that could produce the "-"?