WordNet sense 1 (something kept back or saved for future use or a special purpose):

backlog, stockpile

WordNet sense 2 (an athlete who plays only when a starter on the team is replaced):

second-stringer, substitute

WordNet sense 3 (a district that is reserved for particular purpose):

reservation

WordNet sense 4 (armed forces that are not on active duty but can be called in an emergency):

military reserve

WordNet sense 5 (formality and propriety of manner):

modesty

WordNet sense 6 (the trait of being uncommunicative; not volunteering anything more than necessary):

reticence, taciturnity

WordNet sense 7 (arrange for and reserve (something for someone else) in advance):

book, hold

WordNet sense 8 (give or assign a resource to a particular person or cause):

allow, appropriate, earmark, appropriate