reference
When 
reference is 
cell, the plane is crystallographic,
its parent must be a cell and the plane indices must be indicated.
When 
reference is 
atoms, the plane is atomic and
must contain at least three non colinear atoms defining a polygon.
Example: <plane ... reference="cell"/> (default)
Allowed values: cell, atoms (optional)
h, k, l
Set the indices 
h, k, l defining a crystallographic plane.
The three indices cannot be all zero. An error is shown when these 
indices are not indicated for a crystallographic plane or when they 
are indicated for an atomic plane.
Example: <plane ... h="1" k="0" l="0"/> (no default)
Allowed values: integer (required, coupled)
vectors
Set the type of cell vectors, 
conventional or 
primitive,
used to determine the crystallographic plane with 
h, 
k, 
l indices. When the lattice is primitive there is no difference 
between 
conventional or 
primitive cell vectors.
An error is shown when 
vectors is set for an atomic plane.
Example: <plane ... vectors="conventional"/> (default)
(defined in GAMGI_PHYS_PLANE_VECTORS)
Allowed values: conventional, primitive (optional)