The default account type of Customer is set in class LedgerJournalEngine_CustPayment method initValue().
void initValue(LedgerJournalTrans _ledgerJournalTrans, LedgerJournalTrans_Asset _ledgerJournalTrans_Asset = null)
{
super(_ledgerJournalTrans);
this.initOffsetFromLedgerJournalTable(_ledgerJournalTrans);
_ledgerJournalTrans.AccountType = LedgerJournalACType::Cust;
_ledgerJournalTrans.SettleVoucher = CustParameters::find().AutoSettle ?
SettlementType::OpenTransact : SettlementType::None;