I haven't something to test but this is what I found online :
Passthrough IDE (raw). Use mode only for remote client device access.
You can write or burn a remote CD only through pass-through mode access.
Passthrough mode is not functional for local host CD-ROM access.
Emulate IDE. Use to access a host CD-ROM device.
The host CD-ROM device is accessed through emulation mode.
In emulation mode you can only read a CD-ROM from a host CD-ROM device.
If you have a CD device on your ESXi host you can use Emulate IDE mode to read it.
This gives you a functionality of just read.
If you have an ISO on a CD Drive on a Desktop from which you are accessing VM using the vSphere client, you will have to use Pass through mode. This gives you the functionality of both read and write(burn).
Read This blog post for better understanding.
http://kunaludapi.blogspot.in/2012/09/vm-settings-cddvd-drive.html#.Uv8bVfmSx8F