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This command only installs the SP, and does not extract the msp from it as documented:
AutoCAD_2015_SP2_64bit.exe / e AutoCAD_2015_SP2_64bit.msp
I have run this in a command window on two seperate computers. Instead of extracting it updates AutoCAD !
Oops, my error was the space '/ e' in the extraction parameter.
I still don't like that it just updated AutoCAD instead of failing.
Make sure there is no space between the / and e.
AutoCAD_2015_SP2_64bit.exe /e AutoCAD_2015_SP2_64bit.msp