The inequality |z−i|<|z+i| represents the region
Re(z)>0
Re(z)<0
Im(z)>0
Im(z)<0
|z−i|=|z+i| represents the real axis.
As z=i satisfies |z−i|<|z+i| , we get
|z−i|<|z+i| represent Im(z)>0