NLS_CHARACTERSET is used for CHAR, VARCHAR2, CLOB columns and NLS_NCHAR_CHARACTERSET is used for NCHAR, NVARCHAR2, NCLOB columns.

One can find the value of NLS_NCHAR_CHARACTERSET used by the database by running the following SQL:
select value from NLS_DATABASE_PARAMETERS where parameter=’NLS_NCHAR_CHARACTERSET’;

VALUE
———————-
AL16UTF16

The possible values for NLS_NCHAR_CHARACTERSET can be UTF8 or AL16UTF16.