In business management:
In accounting: the BookingEngine
In CRM: the ActionEngine
By possessing the module corresponding to these engines, you obtain the methods (functions) of the engine and you can program customised applications yourself.
You can write a code or a customised program within Mercator using the engine's functions.
You can write a code for an application which is outside Mercator but uses its engines. For example, this is the case if you write an ASP.net application linked to Mercator.