I did a bit of web research for your question, but turned up very little in the way of rigorous sources. However, this link tells us that a click (also klik or klick) is generally used to mean a kilometre, or occasionally a kilometre per hour (two quite different things!).

Neil

Dan Berger adds:
I am a US Army veteran. "Klick" is short for "kilometer" (and is convenient enough that some Germans have picked it up from the Americans), and is also used for "kilometers per hour." Context can usually tell you which is which.

The usage is not, of course, official, but it IS universal.