What is the difference between vested and sellable quantity of RSUs? I've read that it's withheld for what's owed in taxes, but it sounds like I *also* still have to pay taxes (long or short term) whenever I sell, so am I getting taxed upon the receipt AND the sale of these shares?
On vest, you pay taxes based on your annual tax bracket just like any other income. On sale, you pay taxes on the profit. This percentage varies depending on how long you held before selling.
Gotcha, so withheld shares are essentially just income tax, not capital gains?
The taxes you pay when you sell are only on the profits you made i.e. sell price - acquired price.