There are many methods of distributing points within a volume.   For simple geometries refer to the createpts command.  This example uses the regnpts command which, although more complicated, provides greate flexibility.  Points are distributed within regions using Cartesian, cylindrical or spherical coordinates by constructing rays that travel through regions and distributing points along these rays. For this example, points are distributed using Cartesian coordinates. The rays are specified by defining a set of points and a plane. For each point in the set, a ray is constructed normal to the plane passing through the point. In general rays are constructed in sets, each set is specified by a single plane and a set of points. The createpts command is used to create the points. The regnpts command is used to specify the plane, to specify the region, and to specify the number of points to be distributed along the rays. The points and the plane should lie outside the enclosing volume and on opposite sides. The normal to the plane should point toward the point. As rays are created, if they do not pass through the specified region, no points are distributed. Points may be spaced evenly along the ray or they may be spaced according to a ratio. The following commands will place points in the unit cube. 

* create 25 points (5x5x1) in a plane above the unit cube
* place points on the boundaries   in the x and y directions (1,1,0) 
createpts/xyz/5,5,1/0.,0.,1.1/1.,1.,1.1/1,1,0/ 
* give the points defined by the createpts command   the name, rayend 
pset/rayend/seq/1,0,0/ 

* create rays between points in rayend and the plane below the cube 
* distribute 3 points along these rays in the region top 
* add one point at the upper external boundary for each ray
* will get 4 points total along each ray in region top 
* "pset,get,rayend" refers to all the points named rayend 
* the three points: (0.,0.,-.1), (0.,1.,-.1), (1.,1.,-.1)
* define a plane whose normal points toward the rayend points
regnpts/top/3/pset,get,rayend/xyz/0.,0.,-.1/0.,1.,-.1/1.,1.,-.1/0,0/  

* distribute 4 points along these rays in the region bottom
* add one point at the lower external boundary for each ray 
* add one point at the material interface for each ray since 
* bottom contains the interface - a total of 6 points for each ray. 
* points will be distributed such  that the ratio of distances between 
* any two consecutive pairs of points is 0.6, traveling from the source
* of the ray (the plane) to the rayend.
regnpts/bottom/4/pset,get,rayend/xyz/0.,0.,-.1/0.,1.,-.1/1.,1.,-.1/1,.6/ 

Other versions of the regnpts are appropriate for cylindrical and spherical geometries. For cylindrical geometries the createpts command specifies points in a cylindrical shell outside the volume. The regnpts command specifies a line (usually the cylinder axis), and the rays are constructed normal to this line and containing one of the createpts points. For spherical geometries the createpts command specifies points in a spherical shell outside the volume. The regnpts command specifies a point (usually the center of the sphere) from which rays are constructed to the createpts points.