Very often, CGM users try to use generic CGM files in WebCGM viewers without success. Here is an overview what you should watch out for.
Generic CGM files without graphical objects
Most commercially available viewers let you display such files without problems.
CGM files with graphical objects (grobjects)
An ATA GREXCHANGE file is not a WebCGM, although the definition of the graphical objects is almost identical in both profiles. The major difference is that WebCGM allows you to specify a link, and ATA not.
CGM files with private application structures
Files that contain private application structures can usually be displayed in viewers of that vendor only. Although the visual image may be shown correctly by other viewers, the intelligent content may not be understood. Some viewers are restricted to display only true WebCGM files. They will reject all other kinds of CGM files.