>What is difference betwen variables (LOCAL, PRIVATE, PUBLIC) and PARAMETERS?

Parameters are variables or values that are passed to a procedure, function, or method.

Local, private, and public are keywords that refer to variables. LOCAL and PUBLIC create variable references with initial values of .F. PRIVATE hides a variable reference that may, or may not, be in the calling program. Try the following:

CLEAR
PRIVATE BAR
BAR = "Goodbye, Cruel World!"
?BAR
=helpme(bar)
?BAR

FUNCTION helpme(t_bar)
  PRIVATE BAR
  LOCAL foo
  PUBLIC foobar
  BAR = "Hello, "
  t_bar = "World!"
  ?BAR
  ??t_bar
ENDFUNC