EMU is Electric Multiple Unit which mainly runs in the Suburban sections of major cities.
MEMU is Mainline Electric Multiple Unit which runs on mainline generally for distances more than the suburban sections. But there are some exceptions to this definition as can be seen in the NDLS-MTJ section where 2 EMU's run daily between SSB-MTJ which is beyond the suburban section. Same is the case with NDLS-ALJN EMU. On the other hand MEMU runs on the NDLS-PNP section which is beyond the suburban section limit.