I am not sure what you are asking, but maybe I can guess?
(1) If using Creo manufacturing, you use your Creo model(s) to define the part you want to make and often the part (or block) you are making it from. You define a set of sequences to cut the surfaces of the part, drill holes, etc. Creo will output these sequences, usually grouped together into what it calls an "operation" as a program file (.ncl). This program file is not, as far as I know, useful for any machine. To be able to use that program to drive a CNC machine, like a 3 axis mill, it needs to be converted to G-code, with a post-processor. The post-processor is only used for a specific brand (like Matsuura, Heidenhaim, etc.) and model of CNC machine. Every machine has its own special format and requirements for the G-code it uses. G-code is very similar from machine-to-machine, but there are a lot of specific settings and codes that have to be right for the program to work.
(2) If you make changes to the Creo model you usually can create the .ncl file again, post-process it, and it should be okay. If you make major changes, like add another hole, or cut a pocket, you will have to add or change sequence definitions to make the program create the correct new part geometry. The G-code is not automatically updated to show the new changes.
