Short answer: Premium tax credits lower your monthly premium; cost-sharing reductions lower what you pay at the doctor (deductibles, copays, out-of-pocket max). CSRs apply only to Silver plans.
The two subsidies target different costs. A premium tax credit reduces the premium you pay each month and is available across metal tiers. A cost-sharing reduction lowers your point-of-care costs; deductible, copays, coinsurance, and out-of-pocket maximum, but only on Silver marketplace plans and only for lower-income enrollees. Many eligible members receive both at once by enrolling in Silver.