Funds in the reserve account should only be used for the items (a.k.a. “reserve components”) listed in the HOA’s reserve study.
Preferably the reserve study was completed in the last five years and updated annually.
A reserve component is going to be any infrastructure (or possibly service) with a life expectancy (when new) of at least one year and for which the HOA has the maintenance responsibility. I mention “service” because, for example, the several thousand dollar cost of a reserve study is typically included as a “component cost” in the reserve study.
Bylaws, CC&Rs and state statutes sometimes place restrictions on the use of reserve funds.