Short answers, pulled from the story.
A military reserve force is an organization whose members hold both military and civilian occupations and are not normally kept under arms. Their main role is to be available when the military requires additional manpower. Most nations treat reserve forces as part of their permanent standing armed forces, allowing peacetime military expenditures to be reduced while maintaining war readiness.
The Krumpersystem was a Prussian military innovation introduced by reformer Gerhard von Scharnhorst after Napoleon capped the Prussian army at 42,000 men at the Treaties of Tilsit on the 9th of July 1807. Recruits received brief training and were returned to civilian life, creating a pool of trained soldiers who did not count against the military ceiling during peacetime. The Imperial German Army retained the system into the First World War.
Colombia, Israel, Norway, Singapore, South Korea, Sweden, and Taiwan all make reserve service compulsory for a number of years after completing national service. In Switzerland and Finland, citizens who have completed active duty but not reached the legal upper age limit remain subject to mandatory mobilization.
South Korean males who complete national service in the armed forces or in the national police are automatically placed on the reserve roster. They are obligated to attend several days of annual military training each year for seven years.
Military reserve forces are more economically effective than regular troops because they are called up only when needed rather than kept continuously on pay. Reservists often bring professional civilian skills, such as engineering, medicine, and communications expertise, that the military would otherwise need years to develop internally. In peacekeeping contexts, their civilian backgrounds also tend to produce better relations with local populations than career soldiers typically manage.
Russia defines military reserve force in two distinct legal senses. In the narrow sense, reservists are citizens who have signed contracts, been assigned to specific units, and participate in those units' operational and combat activities. In the broader sense, the term includes all citizens who can be mobilized, including those who have signed no contract and can be deployed against their will.