Field in endpoint, but not screen or DAC schema?

Hi @jhouser , this article lists the changes in the entities and fields of the endpoints. These fields may have different names than the DAC fields.
If you need to find out the changes in DACs, you need to review the list of changes in the Release Notes for Developers. You can find a link to this document for 2024 R1 on the following page: 

If you need to find out to which DAC field the endpoint field relates, you need to go to the Web Service Endpoints (SM207060) form, select the needed endpoint, find the needed entity in the list on the left, and on the Fields tab see the mapping to an element of the respective Acumatica ERP form.
 

You then may need to go to the form and use the Element Inspector (https://help.acumatica.com/(W(3))/Help?ScreenId=ShowWiki&pageid=5f9d2d33-1a33-4edd-8629-3100d2020335) to find the exact DAC and field for the element specified in the mapping.
For the field you mentioned, the respective DAC field is SOOrder.ExternalOrderSource. You may not see it in the DAC Schema Browser of your instance if the RetailCommerce feature is turned off in the instance.