You should use the "Elevation Label 19" label, which was created specifically for this purpose. The "Generic Label 19" can show pretty much all data that any of the other labels can show, but you need to set more parameters to display only what you need.
Also, the Core option applies only to Slabs since they are the only element type where the skins are horizontal (in Walls they are vertical, in Roofs/Shells, they are mostly slanted so they are different at different points of the Roof/Shell).
As for the value displayed, try to set the vertical Elevation value of your Floor Plan to 100 in your Story Settings Dialog, then set the "Elevation Label 19" label to display values relative to "Project Origin".
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