The IIT white matter atlas is a connectivity-based atlas. For its construction, each gray matter label of the IIT gray matter atlas (with Desikan labels) was used as a seed and all other gray matter labels combined were used as targets for probabilistic tractography on the IIT HARDI template. 

The IIT white matter atlas contains four-dimensional (4D) white matter labels. This means that each white matter voxel is assigned a list of gray matter label-pairs with the most probable connections through that voxel. The reason behind the 4D labels is that multiple fiber populations often share the same voxels, and therefore white matter voxels must be given the chance to have multiple labels.

To learn more about how you can interpret the information contained in the white matter atlas see "HOW-TO-use-the-WM-atlas" document in the Docs section.