CEC = Mainframe.  MainView Explorer(s) may run on CECs (maybe in LPARs thereof?).
LPAR = Mainframe Partition (MFPart to ADDM)
       Operating system instances like hosts
       Each LPAR runs a BMC MainView instance.

http://www.slideshare.net/abhi1112/mainframe-architecture-product-overview

I had a great HTML architecture summary but can't find it.

Operating Systems
    z/OS.  Unix-compatible. Runs on Mainframes.
    IBMi   Runs on Power Systems and PureSystems