fix wall error

Well. I have a stupid question for this fix wall command.
If I had a specific coord setting, such 290 in a 300-angstrom cube along the z axis, what is the meaning of the face setting lo or hi? There seems no need to have the zlo or zhi setting, just a z axis setting would be ok. But If I switch from zhi to zlo, there is the error again, particle on or inside the wall.
Or is my understanding of the coord setting right?
Thank you.