Often in shader programs you need to access some global state, for example, the current modelviewprojection matrix, the current ambient color, and so on. There’s no need to declare these variables for the built-in state, you can just use them in shader programs.
Matrices (float4x4) supported:
UNITY_MATRIX_MVP: Current model*view*projection matrix
UNITY_MATRIX_MV: Current model*view matrix
UNITY_MATRIX_V: Current view matrix.
UNITY_MATRIX_P: Current projection matrix
UNITY_MATRIX_VP: Current view*projection matrix
UNITY_MATRIX_T_MV: Transpose of model*view matrix
UNITY_MATRIX_IT_MV: Inverse transpose of model*view matrix
UNITY_MATRIX_TEXTURE0 to UNITY_MATRIX_TEXTURE3: Texture transformation matrices
Vectors (float4) supported:
UNITY_LIGHTMODEL_AMBIENT: Current ambient color.