The National Character set (NLS_NCHAR_CHARACTERSET) is used for data stored in NCHAR, NVARCHAR2 and NCLOB datatypes and is a character set which is defined in addition to the (normal) database character set (NLS_CHARACTERSET), which is used for CHAR, VARCHAR2, LONG and CLOB datatypes.
current value :
select value from NLS_DATABASE_PARAMETERS where parameter = 'NLS_NCHAR_CHARACTERSET';
You cannot have more than 2 charactersets defined in Oracle:
The NLS_CHARACTERSET >>>>>>> CHAR, VARCHAR2, LONG and CLOB columns;
The NLS_NCHAR_CHARACTERSET >>>>>>> NCHAR, NVARCHAR2 and NCLOB columns.
From 9i onwards the NLS_NCHAR_CHARACTERSET can have only 2 values: UTF8 or AL16UTF16 who are Unicode charactersets
The NLS_NCHAR_CHARACTERSET defaults to AL16UTF16 if nothing is specified, this is the recommended value.
1) NCHAR
2) NVARCHAR2
3) NCLOB
https://support.oracle.com/CSP/main/article?cmd=show&type=NOT&id=276914.1
https://support.oracle.com/CSP/main/article?cmd=show&type=NOT&id=260893.1