You're trying to prevent a double dip and/or avoid double taxation.

If it was already taxed, then it comes out not taxable. Examples are Basis (nondeductable Trad IRA), or nondeductable Roth contributions (+ Roth earnings, by regulation, are not taxable). Moving any combination, such as Trad Basis + Earnings (which were never taxed), is pro rata taxable.

If it went in pre-taxed, it comes out taxable. Pre-tax through payroll deduction, or pre-tax meaning deductible/deducted on a tax return, or even rolled over from 401(k) to Trad IRA, etc.

If there is no Basis, then everything Out is taxable, even if not into the taxpayer's hands, unless it simply moves to a similar account type.

401(k) to Trad IRA = not taxable (not a conversion, but a transfer)

401(k) to Roth IRA = taxable (conversion)

Roth 401(k) to Roth IRA = not taxable (transferred from employer plan to personal retirement account of the same type)

And remember the new rules include that the employer can now put the match into the Roth 401(k), and that would have already be taxed through payroll. That's a new provision.