JS or css file behind the argument with the reason _javascript tips

Frequently encountered in the page in the download of JS and CSS files with parameters, such as:

<script type= "Text/javascript" src= "jb51.js?version=1.2.6" ></script><link rel= ' stylesheet ' href= ' base.css?version=2.3.3 ' type= ' text/css '/>
There are two possible uses of parameters,

The script does not exist, but it is dynamically generated by the server, so there is a version number to differentiate.
The client caches these CSS or JS files, so each time you upgrade the JS or CSS file, change the version number, the client browser will download the new JS or CSS files
The second is the most, and there may be two of them at the same time.

The version number, can be a random number, or it can be an incremented value, a large version of the version of the way, or according to the script generation time to write, such as 20080727182553 is accurate to generate script seconds, and 2.3.3 is a large version of the small version of the way.