1. All variables are declared and initialized only in the DECLARATIVE SECTION.
    a) Declarations Allocate storage space for a value.
    b) Declarations can be assigned with a initial value and can be imposed with a NOT NULL Constraint.
2. Assign new values to variables in the executable section.
    a) The existing value of the variables is replaced with a new one.
    b) Forward reference are not allowed.
3. Pass values into PL/SQL subprograms through parameters.
    a) There are three parameters modes, they are IN(the default),OUT and IN OUT.
4. View the results from a PL/SQL block through output variables.
   a) Reference variables can be used for input or output in SQL data manipulation statements.