The reason for parameters following the JS or CSS file

JS and CSS files loaded on pages often contain parameters, such:

<SCRIPT type = "text/JavaScript" src = "jb51.js? Version = 1.2.6 "> </SCRIPT> <LINK rel = 'stylesheet 'hrefpolic'base.css? Version = 2.3.3 'Type = 'text/CSS '/>
There are two possibilities for using parameters,

The script does not exist, but is dynamically generated by the server. Therefore, a version number is provided to indicate the difference.
The client caches these CSS or JS files. Therefore, after each JS or CSS file is upgraded, the client browser will download the new JS or CSS file again after the version number is changed.
The second case is the most, or both.

The version number can be a random number, an incremental value, a large version or a small version, or a script is written based on the script generation time. For example, 20080727182553 is accurate to the second when the script is generated, 2.3.3 is the method of using a major version or a minor version.