User Defined Fields (UDFs) can be added to most windows in Orchestrated (Sales/Purchasing documents, production, Item Master Data, Business Partner Master Data, etc).
You can create a toggle field, a text box, a date box, as well as a numerical field or alphanumerical field. Once the field type has been selected, however, it cannot be edited.
This video covers the process of adding a User-Defined Field (UDF) to Orchestrated:
These can be used to add a text box, number, date, etc to different tables in the system. UDFs are useful for adding extra notes about an item or Business Partner, and can also be used in queries.