The cell orientation is given by two vectors, normal to each other,
defining an axes set. The reference orientation is given by two 
vectors, normal to each other, defining an axes set.
This alignment consists in rotating the cell, so the first cell 
vector is aligned with the first reference vector, and the second 
cell vector is aligned with the second reference vector.
Cell
The first line defines a vector normal to the plane indicated
by entries 
h, 
k, 
l. The second line defines
a vector parallel to the direction indicated by entries 
u, 
v, 
w. The two vectors must be normal to each other,
otherwise an error is shown. The two vectors are perpendicular
when the Weiss law is obeyed: hu + kv + lw = 0.
The 
h, 
k, 
l and 
u, 
v, 
w
cell coordinates can be given in terms of 
Conventional
or 
Primitive vectors, using the 
Vectors menu.
Reference
The reference can be the current layer (the default) or another 
cell. To select a cell as reference, press 
Cell and click 
on its visual representation (or write its identification directly 
on the enabled entry).
When the reference is a cell, the orientation is defined
by a plane and a direction, or vice-versa. When the reference 
is the layer, two directions must be entered, based on the layer 
orthonormed axes.
The first line defines a vector normal to the plane indicated
by entries 
h, 
k, 
l, or parallel to the 
direction indicated by entries 
u, 
v, 
w.
The second line defines a vector normal to the plane indicated
by entries 
h, 
k, 
l, or parallel to the 
direction indicated by entries 
u, 
v, 
w.
The 
h, 
k, 
l and 
u, 
v, 
w 
reference coordinates can be given in terms of 
Conventional 
or 
Primitive vectors, using the 
Vectors menu.
When the reference is the layer, only the option 
Conventional
is available.
The two vectors must be normal to each other, otherwise an error
is shown. When the reference is a cell, the Weiss law must be
obeyed: hu + kv + lw = 0.
When the reference is the layer, the layer axes are equivalent 
to the axes of a cubic primitive lattice, where planes are 
perpendicular to directions with the same indices, so one of 
the two directions can be seen as representing a plane with 
the same indices, thus the Weiss law must again be valid: 
u1u2 + v1v2 + w1w2 = 0.
Pressing 
Ok on the second level dialog, the 
Align
information is saved but the actual rotation is calculated only 
after pressing 
Ok on the first level dialog, to ensure
that changing later some cell parameters will not produce
wrong results.
To disable the 
Align information, open again the 
second level dialog, which now shows the previously 
entered data, and press 
Cancel.