"Basis" for a traditional 401k, that only refers to any after-tax contributions to a traditional 401k.
Many people don't have any after-tax contributions, only pre-tax contributions, so if you never made any after-tax contributions, then all of the conversion will get taxed.
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Some of us (myself) had some after tax contributions to a traditional 401k plan due to limits on what pre-tax amount they could contribute in some prior years. In that case, that "basis" (after-tax) contribution amount may partially tax-free during the conversion to ROTH. (or even more rarely, all tax-free for the after-tax amounts contributed before 1987).
But your 401k administration representative should be able to tell you if that is the case for any of your conversion $$, and how much is not taxed (if any) when you discuss with them the amount you plan to convert.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
