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Due to the complexity of modern video and audio encoders there are many options and settings that may vary from one application or piece of hardware to another. In many cases these settings, are divided into proflies to allow video encoders to know what settings are supported for particular hardware or software playback configurations.
While profiles are traditionally used by standards group like the Motion Pictures Experts Group (MPEG), its become increasingly common in recent years for profiles to be created for certification programs from companies like DivX and Nero. While the profiles used by these companies are similar to the official profiles for the MPEG-4 standards used, they don't correspond exactly. In other words the resolutions, audio formats, and bitrates allowed by a particular profile may be more limited than the MPEG-4 profile being used.
In addition to profiles, MPEG encoding uses levels, which are specific limitations within a given profile. For example, HD DVD and DVD both use the MPEG-2 Main Profile, but DVD uses the Main Level, while HD DVD uses High Level. Both support B frames and interlaced encoding, but due to the level used DVD only allows resolutions up to Full D1 (720x480/576) while HD DVD's High level allows resolutions up to 1920x1080.