The keyword Self is used in the definition of a Variable to refer to that variable.
Usually, a variable cannot refer to itself -- because it creates a cyclic dependency -- so you cannot use Self in common expressions. One exception is in the Dynamic function when referring to Self at a preceding Time period, e.g.
Variable X := Dynamic(10, Self[Time - 1]*(1 + growth))
The other exception is in IntraTable that allows cells of a table to refer to each other, using Self.