When you are granted stock options or restricted stock units (RSUs), they are typically subject to a vesting schedule, meaning that you have to meet certain conditions before exercising options or receiving the shares of stock. These conditions can be found in the terms and conditions set by the company in the company equity plan.
In some countries or jurisdictions, withholding for stock options and RSUs typically occurs when options are exercised or shares are vested. The company will typically withhold a certain number of shares (or cash equivalent value) to cover the taxes that will be owed on the transaction. The specific amount of withholding will depend on your tax situation and the stock's fair market value at the time of exercise or vesting.
It's always recommended to consult with a tax advisor to understand the tax implications related to your specific situation.