Is the 'Order Number' a mandatory field in all XML trade confirmations?

Is the 'Order Number' a mandatory field in all XML trade confirmations?

The eCM Standards 4.4 state:
Order Number

Conditional

If ‘TransactionType’ is a ‘Financial Transaction’, or “PHYS_INX” or “OPT_PHYS_INX” then this field:
a) is Mandatory and Key if the deal has an Order Number
otherwise
b) must be omitted


This means, for the ‘Order Number’ field, if the field is present then the information must be filled in. For example you cannot include: <OrderNumber></OrderNumber> within the XML document or this will break the Business Rule. However for the trade to be accepted in the CMS, it does not require an ‘Order Number’ to be present. Therefore you can remove this field in the XML.