When box 2a is empty and box 2b is checked (taxable amount not determined), this means that the plan administrator does not believe that they have enough information to calculate that amount of the distribution that is taxable.
As you proceed through the interview, TurboTax will ask you a number of questions so that it can use the Simple Method (normally) to calculate the taxable amount. Why is this necessary? Because if you have after-tax dollars in the plan (called "basis"), then this amount is spread out over a period of time and returned to you. That is, if a typical monthly payment is $1,000, then $50 of that payment may be return of basis and not taxable, so the taxable amount for that month would be $950.
Prior to a few years ago, plan administrators were not required to track such after-tax contributions when the plan was bought or sold.
If you have no after-tax dollars in your plan, then the taxable amount will be the same as the distribution...but TurboTax has to ask for your benefit (many people end up paying tax on such dollars twice because they don't know any better).
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