Hope this Info from a vendor helps

Gerber files
A PCB is fabricated as a series of layers that the manufacturer assembles
into a board through a variety of chemical and mechanical processes. To
fabricate each physical layer in the PCB the manufacturer uses an image of
that layer -- this image is referred to as a phototool. A phototool is a
piece of clear film, with black lines, circles and other shapes forming
exactly the same patterns as the content of that layer in AutoTRAX.

Gerber data is a simple, generic means of transferring printed circuit board
information to a wide variety of devices that convert the electronic PCB
data to artwork produced by a photoplotter. Virtually every PCB CAD system
generates Gerber data because all photoplotters read it. It is a software
structure consisting of X,Y coordinates supplemented by commands that define
where the PCB image starts, what shape it will take, and where it ends. In
addition to the coordinates, Gerber data contains aperture information,
which defines the shapes and sizes of lines, holes, and other features.

Gerber Format, which is the format in which Gerber data is expressed,
actually is a family of data formats that are subsets of EIA Standard
RS-274D. Extended Gerber Format, which is also called RS-274X, provides
enhancements that handle polygon fill codes, positive/negative image
compositing, and custom apertures, and other features. RS-274X also
encapsulates the aperture list in the header of the Gerber data file and
therefore allows files to pass from one system to another without the need
to re-input the aperture table. RS-274X produces a variety of Gerber data
called X data.

RS-274X is a superset of the EIA Standard RS-274D format. RS-274X supports
some of the parameter data codes (G codes) and aperture codes (D codes)
contained in RS-274D, as well as codes referred to as mass parameters. Mass
parameters are plot parameters that define characteristics that can affect
an entire plot, or only specific parts of the plot, called layers. Mass
parameters extend the capabilities of Gerber Format. Their presence makes
the Gerber data X data. RS-274X is maintained by Gerber Systems Corporation
(GS), a leading supplier of CAD/CAM systems, large-area plotting systems,
and precision cutting systems since 1965.

An RS-274X plot file is a file consisting of RS-274X parameters and standard
RS-274D codes which, when correctly interpreted, result in an image that may
be displayed or plotted.

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