So, when your RSUs vest, you have realized wage income in the amount of the vest. As a result, the company needs to send withholding in to whatever jurisdictions tax your income. There's two questions then: how much, and where do they get the money.
For US federal income tax, withholding from equity is generally treated with the supplemental (bonus) withholding rates; there's an easy way and a hard way, the easy way is 22% for the first $1 million in supplemental wages per calendar year, and 37% after that. The hard way is mush it in with your regular payroll withholdings (until you reach $1M of supplemental wages, and then 37%).
For california, the employer can do the easy way and supplimental wages from equity are fixed at 10.23% or they can do the hard way like in federal.
Chances are, payroll is going to use the easy way, and your withholding changes aren't going to matter for supplimental wages.
Part two is that they have to get the money to pay the withholding from somewhere. Most companies will do net share withholding, so you just get about 2/3rds of the shares (assuming california), cause the other 1/3rd was bought by the company. But, if there's a market, they could do an automatic sale for withholding. Or they could ask you to pay the withholding, or they could give you a cash bonus, and take it for withholding (but they have to gross-up that bonus, because it also needs to have withholding paid).
For US federal income tax, withholding from equity is generally treated with the supplemental (bonus) withholding rates; there's an easy way and a hard way, the easy way is 22% for the first $1 million in supplemental wages per calendar year, and 37% after that. The hard way is mush it in with your regular payroll withholdings (until you reach $1M of supplemental wages, and then 37%).
For california, the employer can do the easy way and supplimental wages from equity are fixed at 10.23% or they can do the hard way like in federal.
Chances are, payroll is going to use the easy way, and your withholding changes aren't going to matter for supplimental wages.
Part two is that they have to get the money to pay the withholding from somewhere. Most companies will do net share withholding, so you just get about 2/3rds of the shares (assuming california), cause the other 1/3rd was bought by the company. But, if there's a market, they could do an automatic sale for withholding. Or they could ask you to pay the withholding, or they could give you a cash bonus, and take it for withholding (but they have to gross-up that bonus, because it also needs to have withholding paid).