The UD field is a way to extend your tables. In E9 (and upgraded versions of E10) they have some generic fields (Number01, CheckBox01, etc) that can be used to store extra data in. The issue with using those, especially if you are doing anything complicated, is keeping track of what “real” values map to the generic fields. By creating a UD field, Epicor is creating a new table - one which you define the schema (field names, types). It then links this to records in the base table.
For example, if you create Part.MyCoolField, if you look in SQL, you’ll notice a new table called Part_UD that has records that link back to the part table. All of this linking is done automagically under the hood so it’s quite easy to use.
Create a UD field (not sure what form you use in E9 to do this)
Regen the proper database (instance) in Admin Console
Recycle the proper appserver in Admin Console.