Using External Reference IDs
You can use this function to enter external reference IDs that do not exist as master data in your SAP system, in quotations and sales orders at both header level and item level.
Prerequisites
You have made corresponding settings in Customizing. For more information, see Customizing at 
Customer Relationship Management
Basic Functions
External References
Define External References
, Define Reference Profiles
, and Assign Reference Profiles to Transaction Type
.
Features
The Additional External References
assignment block is available in the sales order only if you have made the necessary Customizing settings. You can enter and save external reference IDs in this assignment block.
The external references are also available in data exchange with other systems. However, you cannot use the reference IDs to navigate to external systems to make corresponding entries there.
If you enter an external reference ID at header level, it is automatically copied at item level for new items. You can manually change this setting, however.
Example
When you sell products to dealers who then sell them on to end customers, you can store the IDs of transactions between dealers and end customers, for example order IDs or invoice IDs, as external reference IDs. If you are required to specify a warranty ID for repairs to be carried out on a warranty, you can store it as an external reference ID.