What is meant by the system or resource's reserve capacity?

The reserve capacity of a system or resource is the buffer of performance or capabilities that is not used under normal operation but can be engaged during increased load or in case of failure of main components.

For example, if a server typically operates at 60% of its capacity, the remaining 40% is reserve capacity, which allows handling peak loads without performance degradation.

In iOS development, reserve capacity may refer to memory, processor resources, or network bandwidth that an application can utilize when necessary without causing crashes or slowdowns.