The second NDC segment, three or four digits, identifying one specific formulation: a given strength in a given dosage form.
The product code is assigned by the labeler, not the FDA, and is unique only within that labeler's range. Product code 4132 means nothing on its own; 0169-4132 means something.
A change of strength, dosage form or formulation requires a new product code. A change of package size does not — that is the third segment.