The old GPM was used to post CLS files.
GPM is no longer maintained and will vanish in the future.
In addition using UGpost, you have much more information at hand, than a CLS file contains.
I would suggest to not go backwards and use CLS files.
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I agree with Stefan. The only situation where posting cl files makes sense is when other cam sytems in addition to NX are used and a common post is developed. ICAM sets up a right click postprocess launch out of NX (their customization) which creates the cl file and launches their postprocessor exe. You could also modify or add to the cl post tcl code to launch an exe when the cl file is done being written.