The difference between RS274D and RS274X gerber formats is the following :
RS274D Standard Gerber was the first version of the PCB file format produced by any CAD (Computer Aided Design) system. This file format known as Gerber is used as input to any Photolithographic Computer Aided Manufacturing (CAM) system inorder to physically realize the design. Once the bare board is fabricated the Gerber file is not needed anymore in PCB Assembly of components onto the board. RS274D Standard Gerber is indeed a file format to drive vector photoplotters which are 2D Mechanical NC machines. It's a simple ASCII format consisting of commands and X, Y coordinates.
RS-274-D file is not an image description because it does not contain all information like the coordinate unit and the definitions of the apertures (Apertures are the basic shapes like fonts in a PDF file.) The aperture file or wheel file is a free-format text file that contains coordinate units and apertures. These apertures are mounted on a wheel, rotated into the light beam intended for human reading, and being set manually by the plotter operator. Since there are no standards for wheel files in RS-274-D, so designer and the plotter operator had to agree their own setups. It is being designed for a manual workflow and only support simple imaging operators thus "stroking/painting" or "vector-fill" are needed in the constructions.
An example of RS274D Standard Gerber file is the following:
RS-274X Gerber also known as extended Gerber or X-Gerber, is a 2D bi-level vector image description format that drives numerical controlled machines like raster-photoplotters. It is a superset of RS-274-D Standard Gerber which contains human readable ASCII format. It is an image description file that consists of a sequence of commands and coordinates. The imaging files are line draw, flash (predefined shapes at a given location), outline fill, and also the planes can be speciefied without the need to "paint" or "vector-fill" like in RS-274-D. combined together. The positive and negative objects can be also combined together. Accordingly, RS-274X is a complete, powerful and clear standard to describe a PCB layer. It's well suited for fast and secure data transfer thus making it reliable for automated workflows.
RS-274X Gerber transfer layer information from CAD to CAM system and also used to drive quality control machines for an automated optical inspection. To transfer drilling information, a Drill data in excellon format is needed as a supplementary data aside from the single conductor or mask layer image.
An example of RS274X Standard Gerber file is the following
Below is the source link :