Top Off system ?

Sometimes the simple things stump me and I am hoping this is one of those
cases. I am installing
a sump and would like to install a top off system. However, I am not sure
how adding water to the sump will
increase the water in the tank. If I have constant flow of water from the
tank through the sump how will a
top off system work. My common sense, which I am hoping is wrong, tells me
that tank evaporation will not
impact the sump level until it goes below the water returns in the tank (
drilled tank ). Also water evaporation in the
sump will not impact the tank level because the water flow is constant. I
must be missing something here.
Can someone explain how a top off system works or point me to a site that
explains the mechanics of the system ?

Thanks,
Jim Reynolds