I know the "N" stands for Nylon.
By reading articles and other code sections i assume if the nylon is broken or damaged then the THHN conductor can not be used in a submerged environment. Another words if you install a THHN/THWN under ground the nylon covering can't be damaged.

I searched the Southwire PDF file on THHN/THWN and the white book but could not find anyhting specific to the nylon jacket.

or the nylon covering is strictly for easy pulling?

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