If you actually looked in the files you would see that the NCL file is a CL-code representation of the NC program. It is the file that is directly put out by Creo. Its syntax is sort of a "universal" one, and is able to be read in for simulating material removal, etc.
The TAP file is the result of post-processing a NCL file. It is coded for a specific machine and its controller, like Dahlih, or whatever brand machine you're trying to send programs to. Usually this file is called "G-Code", too.
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