The site license version of the EV3 Software is necessary when you want to use the LEGO Mindstorms EV3 Education Software on more than one computer.
It includes the LEGO Mindstorms EV3 Education Software, and allows you to install and run the software on all computers located at the address of the purchasing institution.
The setup is different from NXT where schools need to buy both a single license and site license product. The LEGO Mindstorms Education EV3 software AND the site licence are included.