Box Plus/Minus is the successor stat to Advanced Statistical Plus/Minus (ASPM), which used to be hosted on this site. Since Basketball Reference now produces Box Plus/Minus (BPM) for the NBA (and does it for the D-League and NCAA Men’s Basketball also), neither of these stats are now produced on this site.
However, the old pages for ASPM are still located on this site and will remain as is.
The full documentation for Box Plus/Minus is located at About Box Plus/Minus on Basketball Reference.
If there are any questions about Box Plus/Minus calculations, please feel free to ask them in the comments section below. I will be migrating questions about Box Plus/Minus from my comments page and other locations to this page to keep them all together.