At grant you receive a right (unit) to acquire a number of shares. A condition is continued employment and when the vesting period is fulfilled you will receive shares.
There is no liability to tax when you receive the RSUs, taxation occurs when the shares are vested and the restrictions lapse and you receive shares.
If you have received dividends during the vesting period it shall be taxed as ordinary income. Once you have received the shares dividends will be taxed as capital gain.