DATA_OPTIONS=SKIP_CONSTRAINT_ERRORS is a very useful parameter mostly used with table_exists_action=APPEND .

In general, if you are appending data ( table_exists_action=APPEND) to an existing table with a unique constraint and if the new rows violating the unique constraint,
then the import job will fail( even if only one row violates the constraint)

DATA_OPTION=SKIP_CONSTRAINT_ERRORS parameter will help in skipping the duplicate row and import the rest of the rows. It is similar to IGNORE=Y option of classic exp/imp

Here I tried to append data with table_exists_action=APPEND, it failed due to a unique key violation.

impdp dumpfile=unique.dmp directory=EXPDIR table_exists_action=APPEND

Now let us try with SKIP_CONSTRAINT_ERRORS option:

impdp dumpfile=unique.dmp directory=EXPDIR table_exists_action=APPEND data_options=SKIP_CONSTRAINT_ERRORS

We can see, it has rejected the conflicting rows and imported the rest of the data.