Available grid types:

Value Description
regularxy Two-dimensional surface data on a regularly-spaced XY grid. The grid size must be specified on creation (with the initgrid command).
regularxyz Three-dimensional volumetric data on a regularly spaced XYZ grid. The grid size must be specified on creation (with the initgrid command).
freexyz Three-dimensional data not restricted to a regular grid. No grid size specification is necessary.