Function is the criterion that concerns how and when a musician uses a composition. There are some pieces which are reserved for specific styles such as light, filmi , classical, folk or dance music. Function may be thought of as a gestalt of artistic, traditional and stylistic factors. Two forms may be said to be functionally identical if they may be interchanged. Therefore, at the level of function the two are identical, though they may be differentiated at another level.
Structure is the criterion based upon the anatomy of the composition.
One piece may be based upon a phrase repeated three, four or nine times. One piece may be evenly split in two. One may exhibit a symmetry while another may be asymmetric. Some may cover several cycles while others may be restricted to but a few beats. These are the structural considerations of a piece.