reference
When 
reference is 
cell, the plane is crystallographic
so plane indices are required and its parent must be a cell.
When 
reference is 
atoms, the plane is described
by real and integer polygon data inside two <data></data> blocks,
so the plane becomes independent of the atoms used to define it.
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)
model
An atomic plane is always represented as a 
polygon.
A crystallographic plane can also be represented as a
pole, a 
trace (for projected planes), or
a 
vector (for reciprocal lattice planes).
Example: <plane ... model="polygon"/> (default)
Allowed values: polygon, pole, trace, vector (optional)