I think you are asking about the field to define for incremental refreshes.
If it is a file system based source then we have only one choice “Incremental update based on new files”, if it is a source like a RDBMS (Oracle/Postgres) or NoSQL like Mongo then you can define a field with the below restrictions
https://docs.dremio.com/acceleration/updating-reflections.html#full-and-incremental-refresh
Kindly let us know if you have any other questions
Thanks
@balaji.ramaswamy