"Z" in sedimen core data

Dear All,
Can you explain about the “Z” in the sediment core data?
I have a sediment core data file, I have incorporated this data into the “initial condition” in sediment tab. I want to see my data at any point cores. I use the “View Plan” by using “control T” to edit the data of sediment cores. When I look at each point, the “Z” data change. Data “Z” that I input is a bottom elevation / elevation point on the surface of the base reservoir cores. I need an explanation about this. Maybe there was a mistake that I made.
I’ve tried to edit by typing directly in the “Z” button but do not want to change.