After Business Objects bought Crystal Reports and launched their product combined as one release they chose to call it Business Objects Enterprise XI. It was essentially BOE 11, but they didn't want it to be called 11 and promoted the name XI which has since become the name of the structure of the platform.
Next release was BOE XI R2, which was basically a service pack, but the version number went from 11 to 11.5. Then they release BOE XI 3.0 and now BOE XI 3.1 which is release 12.
So to answer your question, XI doesn't stand for anything else than 11, but it is pronounced as x-i .(Roman number of 11) .
And BOXI is a basically reporting tool similar to cognos, SSRS.
Regards,
Pranay