SCOPE AND VISIBILITY OF PL/SQL IDENTIFIERS:

a) References to an identifiers are resolved according to the scope and visibility.
b) SCOPE: Scope of an identifier is that region of a program unit, from which we can unqualified the identifier.
c) VISIBLE: It is the region from which we can reference the identifier using an qualified name.
d) Identifiers declared in a PL/SQL block are considered LOCAL to that block and GLOBAL to all its sub-blocks.
e) If a GLOBAL IDENTIFIER is  re-declared in a sub Block, Both identifiers remain in scope.
f) With in the sub block, only the local identifiers is visible.
g) A qualified name should be used to reference the Global Identifiers.
h) The Same Identifier can be declared in two different blocks, the change in one does not affect the other.