MSCDEX, also called Microsoft MS-DOS CD-ROM Extensions, is a program from Microsoft that allows some versions of their MS-DOS and Windows operating systems to use and control CD-ROMs with ISO-9660 file system. This effectively allows the user to access and use either the physical disc or a disc image (in case of virtualization) as another drive, much like in later versions of Microsoft Windows with built-in CD/DVD drive support.
The program exists in two forms: one is the installable add-on, which the user installs on-top of MS-DOS/Windows, the other is included with some versions of MS-DOS and Microsoft Windows. According to Microsoft's Q123408, versions 2.1-2.2 were never included with any of their operating systems, version 2.21 was included with Windows for Workgroups 3.1, version 2.22 with MS-DOS 6.0 and version 2.23 with MS-DOS 6.2-6.22 and Windows for Workgroups 3.11. The last version appears to be 2.25, used exclusively on boot disks for Windows 95, 98 and Me. These Windows versions include built-in support for CD/DVD drives with additional features, thus the development of MSCDEX has ceased.
The drive letter assigned to the CD-ROM drive varies, depending on the MSCDEX distribution used. For example, if the add-on version is installed, the driver letter will be D: by default (assuming C: is the system partition, while A: and B: are floppy drives). On Windows 9x boot disks without RAMDrive, the drive letter will be set to R:. On boot disks with RAMDrive, letter D: is used by the RAMDrive and E: is set as the CD-ROM drive.
It's also interesting that different versions use different CD-ROM drivers. The add-on version 2.23 uses LG's CD-ROM driver, while version 2.25 on 9x boot disks uses a driver by Oak Technology. The device name also varies, add-on and non-RAMDrive boot disk versions use "BANANA", while RAMDrive boot disk versions use "MSCD0001". The only exception is non-RAMDrive boot disk for Windows Me, which names the device "TOMATO". The device name can be changed by the user, though.