It is very unclear what you are trying to do or expecting to happen. As Stefan said the USING statement has no function than to allow the use of an unqualified class name in the remaining body of the class. I.e., being able to reference just "base" instead of "ProcedureLibraries.base". Actually using the class depends on what you want to do with it. Given that the current example merely defines a variable, has no assignments to or from that variable, and has no methods, it is unclear what such use would be.
Incidentally, while you don't say what your PROPATH is (which is relevant since USING statements are PROPATH relative), one guesses that ProcedureLibraries is on your PROPATH. This would mean no USING statement was required at all. Moreover, it is very bad practice since you will soon have a zillion loosely related files in one directory. I tend to use Module/Function so that, for example, an invoice copy class might be in AR/Invoice/InvoiceCopy.cls. It could then be referenced elsewhere by AR.Invoice.InvoiceCopy or with USING AR.Invoice then just InvoiceCopy.