What is an "un-encrypted pass through device"?

In that KC, it expects the following order:
1. Create a partition for the encrypted block device
2. Format with cryptsetup
3. Create the un-encrypted pass through device
4. Format with a standard filesystem such as ext4
5. Mount the filesystem on the encrypted block device

After step 1, the partition is already created. So, I'm guessing that if this "un-encrypted pass through device" is a real thing, it's not created by creating the partition.

Note that lab 10.1 mentions this also.

I suppose that's logical, as that's the only step that doesn't appear to correspond to a description, and it comes between the other two steps, but can you elaborate a little more on this? How exactly is this a "un-encrypted pass through device"?