If we attach an Inertial frame of Reference with the centre of mass of many particle system then centre of mass in that frame of reference would be at rest or, Vcm=0 , and such type of reference frames are known as centre of mass frame of reference.
Total Linear Momentum of a many particle system is zero in centre of mass frame of reference i.e., pcm=MVcm=0 since Vcm=0.
Therefore C-reference frames are also known as zero momentum reference frames.
Since in absence of any external force the centre of mass of any system moves with constant velocity in inertial frame of reference therefore for a many particle system C-frame of reference is an inertial frame of reference.
Reference frames connected to laboratory are known as L-frame of reference or laboratory frame of reference.