As per my understanding, for the external mapping, if PRPC says that you can use the Obj- methods then you don't need to map any external columns as long as the column names and properties are the same. PRPC is smart enough to automatically map the columns to the properties where the name matches.
In general, we use mapping wizard for external class mapping :
Here are a few details I found regarding external class :
An external class is a concrete class (instance of the Rule-Obj-Class rule type) that corresponds to a table in an external relational database, rather than to a table or view in the PegaRULES database.
No inheritance or rule resolution applies to external classes, even if the class name contains a dash character.
Each external class has an associated database table instance (Data-Admin-DB-Table class) and so cannot be part of a class group.
These methods can operate on an instance of an external class:
You can't create Declare Index rules which have an external class as the Applies To key part.
In some cases, no single property in the external class can serve as a unique handle. To support the Obj-Open-by-Handle method with external classes, PRPC assembles a substitute string from the key columns of the table.
Also, It is preferable to have tables as internal ones because the extra columns (particularly pxObjClass) can prove very useful in processing.