All of their amounts in Trad IRA, SEP IRA and SIMPLE IRA are aggregated together. If they have any of these funds still in any of these accounts at year end, that creates a pro rata conversion when they try to do Backdoor. That's because their nondeducted contribution is Basis (already taxed) and the other amount(s) in other account(s) which are not Basis are considered to have been proportionally converted. You can't select only specific dollars or even only one specific account for conversion. That's what "aggregated" means.