How does condo board determine where and how much to spend?

How do other condo associations determine how much their Board of Directors may spend (from either the Operating or Reserves Account) without owners' approval? If a Board is consider certain non-operating purchases, can it simply determine and approve on its own what amount to spend? What policies can owners' put in place to regulate how much a Board spends? Does the answer differ for repair/replacement expenditures versus non-repair/replacement expenditures, such as installing a sprinkler system or fancier floor tile?