Gates wonders what proportion tax revenue should a state raise from each of the three main tax bases; capital, labor, and expenditure? And how can a state counteract tax avoidance by large companies and wealthy individuals? He believes taxing robots is part of the solution.
In contrast, the robot tax, just like salaries, would be calculated on an amount notionally payable out of revenue and would be payable in the tax jurisdiction in which the robot was located. This would be where the revenue was economically generated and this location would be determined by economics rather than tax considerations.
https://www.businessinsider.com/bill-gates-robot-tax-brighter-future-2017-3