The standard defines several sets of capabilities, which are referred to as "profiles", targeting specific classes of applications. These are declared using a profile code (profile_idc) and sometimes a set of additional constraints applied in the encoder. The profile code and indicated constraints allow a decoder to recognize the requirements for decoding that specific bitstream. (And in many system environments, only one or two profiles are allowed to be used, so decoders in those environments do not need to be concerned with recognizing the less commonly used profiles.) By far the most commonly used profile is the High Profile.