Virtual CD/DVD drives by default support only reading. The medium configuration is changeable at runtime. You can select between the following options to provide the medium data:
Host Drive defines that the guest can read from the medium in the host drive.
Image file gives the guest read-only access to the data in the image. This is typically an ISO file.
Empty means a drive without an inserted medium.
Changing between the above, or changing a medium in the host drive that is accessed by a machine, or changing an image file will signal a medium change to the guest OS. The guest OS can then react to the change, for example by starting an installation program.
Medium changes can be prevented by the guest, and
The identification string of the drive provided to the guest, displayed by configuration tools such as the Windows Device Manager, is always VBOX CD-ROM, irrespective of the current configuration of the virtual drive. This is to prevent hardware detection from being triggered in the guest OS every time the configuration is changed.