In domian initialization or Global initialization window we can define the Volume fraction of two fluids e.g. 0.5. But how two define the region during initilization.

Try using a step function. For example:

step((z-1[m])/1[m])*vof1+step((1[m]-z)/1[m])*vof2

would give you vof1 for z > 1 and vof for z < 1.

Thanks Johnny it worked.