The MB-D15 is a Multi-Power Battery Pack for the D7100.
The MB-D15 Multi-Power Battery Pack can be powered either by six AA batteries (Alkaline or Ni-MH or Lithium), or EN-EL 15 rechargeable Li-ion batteries. To use the MB-D15 with AA batteries, use the MS-D14 AA Battery Holder. When using EN-EL15 batteries to power the MB-D15, use the MS-D14EN Battery Holder.
The MB-D15 is equipped with a shutter-release button, AE/AF lock button, multi selector, and main- and sub-command dials for improved operation when taking photographs with the camera vertically oriented.