Essentials and Advanced include batch processing licenses. Essentials comes with four, and Advanced comes with eight.
To use a batch processing license, you use the -processing command-line option. For example:
xsibatch -processing -script"Processing licenses cannot be used to do any rendering (but they can be used to export .mi2 files).
Processing licenses allow you to have up to eight (Advanced) or four (Essentials) computers running batch processing jobs. Batch processing can includes things like exporting .mi2 files, exporting rib files, baking, committing, asset management, custom render management/submission, dynamics, ... anything in a complex pipeline that requires automation and remote processing, but not rendering.
For more information, see Understanding XSI Licensing on the XSI wiki.