If a cloud computing customer wishes to guarantee that a minimum level of resources will...

A reservation guarantees to a cloud customer that they will have access to a minimal level of resources to run their systems, which will help mitigate against DoS attacks or systems that consume high levels of resources. A reservation pertains to memory and CPU resources. Under the concept of a reservation, memory and CPU are the guaranteed resources, but storage and networking are not included even though they are core components of cloud computing. Software would be out of scope for a guarantee and doesn't really pertain to the concept.