In several places in the DIRSIG inputs or outputs, a multidimensional array will be "packed" into a 1D array, string, etc. for storage.
2D Array Packing
There are numerous places where a 2D array or matrix is used in DIRSIG. These include:

3D affine transforms (a 4x4 matrix) are used:

Mueller Matrix data
Since there are two dimensions of the data, the data can be "unrolled" in two different orders: rowmajor and columnmajor.
Row Major
Consider the following 4x4 matrix:
m00 m01 m02 m03
m10 m11 m12 m13
m20 m21 m22 m23
m30 m31 m32 m33
The sixteen (16) elements in this 2D matrix can be packed into a 1D array using rowmajor order as:
m00 m01 m02 m03 m10 m11 m12 m13 m20 m21 m22 m23 m30 m31 m32 m33
Column Major
Consider the following 4x4 matrix:
m00 m01 m02 m03
m10 m11 m12 m13
m20 m21 m22 m23
m30 m31 m32 m33
The sixteen (16) elements in this 2D matrix can be packed into a 1D array using columnmajor order as:
m00 m10 m20 m30 m01 m11 m21 m31 m02 m12 m22 m32 m03 m13 m23 m33