/*+ APPEND */ hint in serial & parallel mode in oracle.

/*+ APPEND */ hint enables the optimizer to use direct-path insert particularly in insert statements in oracle

In serial mode, direct path can be used only if you include the APPEND hint.

Conventional insert is the default in serial mode. 
It means that NO UNDO will be written. 

UNDO is backed by REDO, so the REDO volume will be accordingly reduced.

Direct path insert is the default in parallel mode. 
In parallel mode, conventional insert can be used only if you specify the /*+ NOAPPEND */ hint.
data is appended to the end of the table,rather than using existing space currently allocated to the table, means after HWM of the current block. 

Direct-path inserts are considerably faster than conventional inserts.

What are the Bind Variables & Substitution Variables?

Bind Variables are also called host variables, which is accessed even after the PL/SQL block is executed.
It can be create using VARIABLE keyword and referenced with a preceding colon-:.

ex.
VARIABLE sal number;
begin
select salary into :sal from emp where emp_id = 11;
end;
/
select firstname , lastname from emp where salary = :sal;

Substitution Variables are used to get user input at run time to avoid hard-coding of values that we can provide run time also.
We can reference the same with in PL/SQL block with symbol ampersand-&.

VARIABLE sal number;
accept empid prompt 'provide emp id' 
declare
v_empid number := &empid; 
begin
select salary into :sal from emp where emp_id = v_empid;
end;
/
select firstname , lastname from emp where salary = :sal;