Restricted Stock Units, Restricted Stock, and even Stock Options, all work differently. I recommend using google and search on:

investopedia rsu

and they even have articles for comparison, such as RSU vs Stock Option or RSU vs RS

The differences are from vesting (date of ownership or documenting basis) and also, notice when the vesting is different than the rights. That's because an employee might have been given the rights over years, the values might have been vesting over a different cycle (creating taxable events) and there might be sales crossing over those "lots" and even sales simply to cover the withholding requirements for the awarded basis that is taxable based on the type of option, award, or vesting. Until you find out the details you need, you won't know what you need to prepare that tax return and you won't know which W2 you use for reference.