The commitment is per hour and never carries over. An hour below it still costs the full commitment; usage above it is billed On-Demand. Stopping or terminating the instance does not stop the plan.
Only instance hours are discounted (plus Fargate 24 and Lambda on Compute plans): EBS 24 , snapshots, public IPv4, data transfer and NAT gateways stay at full price, and RDS 24 needs a separate Database Savings Plan. RIs apply first, then EC2 24 Instance plans, then Compute plans. A plan cannot be cancelled, only returned within seven days (same calendar month, $100 an hour or less).