Reserved instances and on-demand instances are the same when it comes to function. They only differ in how they are billed.

Reserved instances are purchased as one-year or three-year reservations, and in return, you get very low hourly based pricing when compared to the on-demand cases that are billed on an hourly basis.

https://blogs.vmware.com/cloudhealth/aws-reserved-instances-vs-on-demand/#:~:text=The%20difference%20between%20Reserved%20Instances%20and%20On%20Demand&text=The%20only%20difference%20between%20the,of%20an%20On%20Demand%20instance.