Session Sequences in Oracle Database:

Basic Usage
Adding the SESSION keyword during sequence creation causes it to be created as a session sequence. Using the GLOBAL keyword, or omitting the additional clause entirely will create the sequence as a global sequence.
DROP SEQUENCE my_global_sequence;
DROP SEQUENCE my_global_sequence_2;
DROP SEQUENCE my_session_sequence;
-- Global Sequence
CREATE SEQUENCE my_global_sequence;
CREATE SEQUENCE my_global_sequence_2 GLOBAL;
-- Session Sequence
CREATE SEQUENCE my_session_sequence SESSION;
Being the default, global sequences have their current value persisted in the database, so they provide a consistent result between sessions in the database, as shown below.
SQL> CONN test/test
Connected.
SQL> SELECT my_global_sequence.NEXTVAL FROM dual;
   NEXTVAL
----------
       1
1 row selected.
SQL> CONN test/test
Connected.
SQL> SELECT my_global_sequence.NEXTVAL FROM dual;
   NEXTVAL
----------
         2
1 row selected.
SQL>
In contrast, session sequences do not persist their current value, so the current value of the sequence is not retained between sessions.
SQL> CONN test/test
Connected.
SQL> SELECT my_session_sequence.NEXTVAL FROM dual;
   NEXTVAL
----------
         1
1 row selected.
SQL> CONN test/test
Connected.
SQL> SELECT my_session_sequence.NEXTVAL FROM dual;
   NEXTVAL
----------
         1
1 row selected.
SQL>
For session sequences, any references to the CACHE, NOCACHE, ORDER and NOORDER clauses are ignored.
Why Use Session Sequences?
Although we have seen they perform better, session sequences are only sensible for specific circumstances. They should not be used as a replacement for the vast majority of sequences. Two specific scenarios where they may prove useful are listed below.
During load operations to staging tables, where the sequence is used by a single session and the contents of the table will be truncated between uses. As a result, maintaining the sequence value is not necessary.
For surrogate keys in global temporary tables. These are session specific anyway, so there is no danger of problems with the session sequences. This is especially useful in 12c as the combination of temporary undo and session sequences means global temporary tables can be used in a read-only database, like active data guard.