It does both (while there's still spinning generators on the grid, those are exactly the same thing because of how the generators work. When there aren't any on

It does both (while there's still spinning generators on the grid, those are exactly the same thing because of how the generators work. When there aren't any on the grid, they are in principle seperate, but the inverters will still use the frequency as a signalling mechanism, since it's a very robust means of controlling a system as large and complex as a grid).