The Geodetic datum or model of the earth used by VBOX GPS engines is WGS84.
This is a recognised standard model of the shape of the earth that is used to calculate heights above sea level at all places on the Earth.
Referencing geodetic coordinates to the wrong datum can result in position errors of hundreds of meters. Different nations and agencies use different datums as the basis for coordinate systems used to identify positions in geographic information systems, precise positioning systems, and navigation systems.
The diversity of datums in use today and the technological advancements that have made possible global positioning measurements with submeter accuracies requires careful datum selection and careful conversion between coordinates in different datums.