Alphabet stock based award activities

In the Alphabet 10-k, the company discloses in its cash flow statement under financial activites sizable “Net payments related to stock-based award activities” , for example in 2017 they disclose a cash outflow of 4166 million USD in this item.

Upon review of the 10-k disclosures it appears their stock based compensation is based on restricted stock units rather than options therefore i do not see why there should be such a massive cash outflow in the cash flow statement each year.

does anyone know what this could be? i dont see any specific disclosure on this at least in the 10k