How simulink function passes the input parameter to matlab?

Hello, i am trying to get the RMS value from an AC voltage source in simulink (discrete formula)

I'm using voltage sensor and rate transition to discretize, however, i don't understand what the rate transition passes to my matlab function.

Using sample period as 0,00001, i understand i should receive, as output, a vector v(sample).

Thing is: i have to sum the square of all the values collected during one period and divide by the number of samples, but how do i do that?

If the input is called "u", i can't, for example, sum u(1)+u(2), since MATLAB function only recognizes u(1), saying that u(2) is another input

How can i do that?

Thanks