In Program Manager Technical Advice 2015-11 the IRS National Office discussed the application of the §6662A penalty when a taxpayer fails to disclose a listed transaction.

Generally under §6662A, if the taxpayer has an understatement related to a reportable transaction, a 20% penalty will apply to that understatement.  However, under §6662A(c), if the taxpayer did not disclose the reportable transaction as required by IRC §6664(d)(2)(A) the penalty increases to 30%.