The matrix file represent regular X Y nodes. Each node contains a Z elevation value. These data can belong to any coordinate system, in any unit system.
Data can take care of any ellipsoid model.
The pace between each node can take any value. It can be less than one meter, or more than 1000 meters. The elevations stored in each node have values that depend on the final application of the client. The interpolation or extrapolation model used to generate the DTM, depends on the final application. Among the models the user will have access to linear, paraboloid-hyperbolic, fractal, with or without extrapolation.