How to understand the annotation of flow? #50

I think the dimension of each voxel is [2020, 2020, 16, 2]. The first three dimensions represent the division of voxel space, and the two-dimensional information in the fourth dimension represents what meaning. In other words, what does the two-dimensional information in the annotations of flow represent? What do the annotations in the first dimension of [:,:,:, 0] and the second dimension of [:,:,:, 1] mean respectively.