The Matrix API is primarily a file-based API, working with CUBE 7 Matrix files. CUBE 7 Matrix files are HDF5 format matrices.

CUBE Matrix files store multiple square matrices, with dimension that is given by the number of zones, in transportation terms, as zones x zones matrices. It is also possible to associate multiple zonal references to the matrices, to provide a meaningful string or numeric reference to the zone index.

The CubeMatrix is indexed by "int", which is a 32-bit signed int. Technical zone limit is therefore 2³²/2 (maximum integer for the index).