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When i apply standard type of cosmetic thread on drafted surface, depth value is grey out as seen in screenshot available in attachment
You need to select the surface to start the thread from first (end of the cylinder).
Also, it has defaulted to a tapered pipe thread which has a standard length.
No difference. It is just the reference for depth value
If you are trying to create a standard straight thread on a drafted cylinder, you need to create the cosmetic thread then draft the cylinder. Otherwise Creo defaults to a tapered pipe thread.
However some standard not seen without drafted surface such as NPTF and NPF standards.