I'm confusd with the version paramter of WWW.LoadFromCacheOrDownload

What is the parameter used for. How can i generate the value. Is it the build number or something else? There is no detail document on the web site.

My solution is …
Put Version File on server (very small size) . Get it at first, then use it as the version parameter. Whenever upload new asset bundle files, increment it, which I made script for automation. And if the Net is not available, use saved Version.