Channel Separation is an important specification for Phono Cartridges.
Channel Balance is also important.
Of course, other 'qualities' of a cartridge count, but keep in mind, any 'preferred' sound quality of any cartridge's Imaging is aided by these two 'relatively achievable' characteristics.
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Consider that all imaging is Phantom, created solely by the relative volume of L volume to R volume.
Channel separation gives the preamp/amp/speakers help maintaining the intended Phantom Imaging.
Tight channel balance also gives a more precise signal/volume difference at the start. A wider image combined with tight control gives more distinction, sharper perception of particular instrument locations.
When you view a line of cartridges, observe, as price goes up, separation specs get wider, and balance specs get tighter.