Doubts in the interpolation control inside the symbol synchronizer object

I'm trying to understand how the interpolation control works. It says that the interpolation control provides the interpolator with 2 signals, the basepoint index and the fractional interval, but the interpolator only has 2 inputs, the signal input and the one from the interpolation control which is the fractional interval, is this right?

Apart from that, which is the interpolation interval? Is it the symbol interval? Because in the picture shows between 3 symbols.