What is system.js in some of phaser examples?

[indy2005]

Hi,

 

I purchased the book and coding examples, Interphase #1, and I see it first imports a file called system.js.

<script src="../../lib/system.js"></script>

This is not mentioned in the tutorial guide, is it a plug-in or part of phaser? 

I have only ever been importing phaser.min.js.

Thanks

i