English is not my mther tongue.

But this sentence doesnt seem confusing to me. It says RMAN Does not check for completeness when duplicates and gives two possible scenarios (here is the OR)

1. You can duplicate a data tablespace but you dont necessary (you can skip them) need to duplicate the associated indexes if they reside in other tablespaces (here is the BUT NOT)
2. You can duplicate a tablespace containing a single partition, this would be the case where you have a tablespace for a partition