Therefore, the 1.2mm DVD disc is constructed of two 0.6mm discs bonded together with the DVD data sandwiched in the center of the two discs. These variations are not available with the CD format. To support this industry standard, the DVD specification provides codes used to prevent playback of discs in various geographical regions. Each player is given a code for the region in which it's sold. A player coded for one region will not play discs that are not allowed in that region.
Regional codes are optional.
Discs can be made with multiple codes or with out codes altogether. Discs without codes will play on all players, regardless of the country. Regional codes also apply to DVD-ROM systems. However, they only apply to DVD-Video discs, which also play back on ROM drives. There are two widely used copy protection technologies currently available for DVD-Video.