"Sound like, if they open traditional IRA and convert to ROTH immediately and no remaining or other traditional IRA left, then it is tax free

If there is other traditional IRA, it is free only fraction of it"

Yes, that's it.

"Example : taxpayer has 95k traditionally in the account and 5K contribute to traditional IRA which $5k convert to ROTH IRA

Calculation

5k of total (95k+5k)=5%

then tax free is only 5% of 5k= about $500 and non tax free $4500 (I dont have exactly calculation)"

Where people go wrong, is thinking it's the same money, and it's not. It's all Values. You didn't literally pull out the same money.

Assuming all the existing Traditional IRA money is pre-tax, deferred earnings, and there is no Basis across these accounts.

$100k value includes new $5k added post tax, then immediately (to avoid any earnings)...

$5k conversion

$250 is tax free, because it is the pro rata portion that was put in post-tax (it's basis) and the $4,750 is taxable.

But, if I had $45k basis in a Traditional IRA and $50k pre-tax (such as in a retirement account through work) and then I put $5k into a new Trad IRA account post-tax...

Then I "backdoor convert" $5k by using only that new account

That is $100k total and $50k basis = half of the conversion is not taxable and half will be, even though it was all of that new account.

That's where people get it wrong.