One earns (accrues) vacation days in proportion to the days worked. After the cap is reached, it is "use it or lose it" - one cannot accrue vacation days any longer.

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What is the maximum accrual? (collapse)
The maximum or accrual cap is the amount designated for the employee’s category, pay status, and years of qualifying service by Personnel Policies for Staff Members, the Academic Personnel Manual, or the union contract. In most cases the maximum allowable accrual is two times the employee’s accrual for one year.

The maximum accruals (caps) have been set by policy for employees covered under PPSM or the APM and negotiated into union contracts for represented employees. It is a violation of the policy or contract to permit employees to accrue beyond the designated maximums.

https://hr.berkeley.edu/faq/vacation-accrual-maximums