The mandatory fields are those that MUST be there for an invoice to be valid under e-Invoice Standard.The optional ones are those that may be needed for the specific business needs of the seller/business. These have been incorporated in the schema based on current business practices in India.The registration of an e-invoice will only be possible once it has ALL the mandatory fields uploaded into the Invoice registration Portal (IRP).A mandatory field not having any value can be reported with NIL.