It's just whatever happens to end up there? That's why D was typically the CD-ROM: A was the first floppy drive, B the (typically absent) second floppy drive, C the only hard disk, and then D was the next free letter.
On my laptop, D is the SD card slot. On my desktop, it's the 2nd SSD.
On my laptop, D is the SD card slot. On my desktop, it's the 2nd SSD.