The Greyscale Transformation Sequence defines the conversion of a stored image (raw pixel data) into a displayed image. The following diagram (DICOM Standard Part 4, Figure N.2-1) shows the entire sequence of transformation.
The first row of transformations does the actual greyscale transformation, which transforms the raw pixel data to the right value to be displayed. The second row of transformations are the post image processing, applying changes other than greyscale transformations.
More details about above lookup tables can be found in Lookup Tables.