As I understand it, you have a code-G Form 1099-R that reports a $10,300 distribution from the after-tax sub-account in your 401(k) and that distribution was split to two different destinations, with the $10,000 after-tax portion rolled over to a Roth IRA and the $300 pre-tax portion rolled over to the regular traditional account in your 401(k).
Because you rolled this distribution over to two different destinations, in TurboTax you must split the Form 1099-R into two, one for the $10,000 that went to the Roth IRA (with $10,000 in box 5) and the other for the $300 that went to the 401(k) (with $0 in box 5). By indicating that the $300 did not go to any kind of Roth account and showing $0 in box 2a, TurboTax will treat the rollover of the $300 as nontaxable.
