We have never tried to suggest a mapping between relational database concepts and lightstreamer concepts.

As a simple rule:
an item in MERGE mode corresponds to a record in a relational table (possibly obtained through a join) identified by a predetermined unique key,
whereas an item in COMMAND mode corresponds to a whole table (possibly obtained through a join) which supports a unique key.

However, this is only a logical mapping and all the conversion stuff has to be performed by the Data Adapter.

Did I address the question?