Congress currency is based around the idea of an IOU. As we are mainly concerned with objects only, it would make more sense for us to have a bartering system. Also Congress loves catergorising and have a set of shapes we use to value our objects. Each shape on the currency notes represents the amount of objects it is worth and the amount of sides the shape has corresponds with the amount of objects you would recieve in exchange for the note.
For example a circle has one side so it represents one object whereas a triangle has three so it is worth three objects.