The validation checks happen on all applicable form controls in the element (paymentTermSelect, customDates, upfrontAmount).
The 'valid' attribute emitted with the `outputPaymentTermForm` can be used to set and update the 'triggerPaymentTermError' attribute in the host application.See a code example in sn_frontend: https://github.com/sportngin/sn_frontend/blob/master/client-apps/org/send-invoices/send-invoices.directive.js#L46-L48
<payment-terms-selector-element
id="test-payment-terms-selector"
organization-id="214091"
sale-type="order">
</payment-terms-selector-element>
outputPaymentTermForm: