In case anyone is wondering what they mean by "5D": they mean it's a 3D array of marks, each of which has both size and orientation. The 3D array is three 2D layers. According to this definition, a DVD or Blu-Ray disc is "4D" because it can have multiple layers each of which stores 1-dimensional data (pit versus no pit). And a book is at least "7D" because it contains a 3D arrangement of marks, and marks can vary in…
I think Dimension in the data sense of the word, not Dimension in the spatial sense of the word. Multi-dimension data is a thing, and it goes a lot higher than 4, 5, or 7.
Where else is that used? At least to me (and the common layperson), the only usage seems to be from people trying to hype up storage mediums.
also, if we're using that definition, does that mean I have a 7D SSD? The NAND cells are arranged on both the planar axis, and stacked (3d NAND). That's 3 dimensions. Each NAND cell also encodes data using various charge levels, which currently tops out at 4 bits. In total that makes for 3 + 4 = 7 dimensions.