A "Display Device" refers to some piece of hardware capable of displaying an image. Display devices are separated into three categories: analog CRTs, digital flat panels (DFPs), and televisions. Note that analog flat panels are considered the same as analog CRTs by the driver.
A "Display Device Name" is a string description that uniquely identifies a display device; it follows the format "<type>-<number>", for example: "CRT-0", "CRT-1", "DFP-0", or "TV-0". Note that the number indicates how the display device connector is wired on the graphics board, and has nothing to do with how many of that kind of display device are present. This means, for example, that you may have a "CRT-1", even if you do not have a "CRT-0". To determine which display devices are currently connected, you may check your X log file for a line similar to the following:
(II) NVIDIA(0): Connected display device(s): CRT-0, DFP-0
Display device names can be used in the MetaMode, HorizSync, and VertRefresh X config options to indicate which display device a setting should be applied to. For example:
Option "MetaModes" "CRT-0: 1600x1200, DFP-0: 1024x768" Option "HorizSync" "CRT-0: 50-110; DFP-0: 40-70" Option "VertRefresh" "CRT-0: 60-120; DFP-0: 60"
Specifying the display device name in these options is not required; if display device names are not specified, then the driver attempts to infer which display device a setting applies to. In the case of MetaModes, for example, the first mode listed is applied to the "first" display device, and the second mode listed is applied to the "second" display device. Unfortunately, it is often unclear which display device is the "first" or "second". That is why specifying the display device name is preferable.
When specifying display device names, you may also omit the number part of the name, though this is only useful if you only have one of that type of display device. For example, if you have one CRT and one DFP connected, you may reference them in the MetaMode string as follows:
Option "MetaModes" "CRT: 1600x1200, DFP: 1024x768"