Oracle dual query

I have a query like this:

SELECT X.Y( 'p1', 'p2', 'p3', 'p4', '100') 
  FROM dual

I kn开发者_如何学Pythonow what dual is but this part X.Y( 'p1', 'p2', 'p3', 'p4', '100') really makes me wonder. What does this syntaxis mean?

Are X and Y tables or what?

Well, I can see three possibilities:

1. X is a schema, and Y is a function within X's schema.
2. X is a package, either with a public synonym or in the current schema, and Y is a function defined in that package.
3. X is a type, either with a public synonym or in the current schema, and Y is a function defined in the type.

What does the SQL*Plus command DESCRIBE X or DESCRIBE X.Y return?