When you use the -processing flag, you can ignore the “Unknown flag” warning. As long as you see “License information: using [Processing]”, you know that xsibatch is using one of the unlimited processing licenses.
C:\Softimage\Softimage_2010_x64\Application\bin>xsibatch -processing-script hello.js
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Autodesk Softimage 8.0.201.0
=======================================================
License information: using [Processing]
COMMAND: -processing -script hello.js
WARNING - Unknown flag (ignored) : -processing
# INFO : Hello World
eX-SI 