No reference because they are not interchangeable and are both required. Uniformity of the dosage unit refers to the mass or weight of the dosage form (tablets, capsules…). Whereas content uniformity refers to the potency of the drug or API in dosage form (mg, ug…).
It is conceivable that someone leaves the ‘good stuff’ out (API) but the dosage form weighs within specification! Thus, the 2 regulations.