get_elem() is for regular configuration elements and their values.
set_attr() callbacks are required when ConfD has the "attributes" / metadata enabled.
Regular yang tree elements can be annotated with a bit of extras in run-time if needed.
This has to be explicitly enabled in ConfD configuration.
If you don't have
/confdConfig/enableAttributes turned on, you can safely set the callbacks to null...
For details see ConfD user guide, chapter 8 - Configuration Meta-Data.