what raw raster image format in pdf ?

hello,

I understood  from "image xobject in pdf" question from bokhandbok that  an xobject image can be either a jpeg  or a raw raster  format.

But to me,  raw is not a standardized format like DNG or tiff/ep are.

so, where can i find a description of the raw raster image format of the pdf ?

is it related to any existing norms like iso 12234-2, dng, etc.

if not, where can i find a description of this format ?

thanks a lot,

denis