From the WAAS Terms & Definitions link at the bottom of the page:

Vertical Protection Level (VPL). The Vertical Protection Level is half the
length of a segment on the vertical axis (perpendicular to the horizontal plane
of WGS-84 ellipsoid), with its center being at the true position, which
describes the region that is assured to contain the indicated vertical position.
It is based upon the error estimates provided by WAAS.


That certainly clears that up!

You're welcome. Maybe an example will be more helpful. Suppose that your true
altitude above the WGS-84 ellipsoid is 1000 feet, and the VPL is 100 feet. The
altitude as indicated on your TSO 146 WAAS-enabled navigator is assured to lie
between 900 and 1100 feet. The WGS-84 ellipsoid is the mathematical model of the
earth's surface used in GPS position calculations.