Code 6 is a "Section 1035 tax-free exchange." Section 1035 is a financial transaction in which a life insurance or annuity policy is replaced for a new one without any taxable event (Basically, it just means replacing one annuity contract for another annuity contract with identical annuitants.).
Please see this TurboTax FAQ for more information on Box 7 codes on your 1099-R: https://ttlc.intuit.com/replies/4194736
{Edited 2//18/18 | 9:33am]