Meaning of an arrow in the control scheme
A convention is that the normal arrows indicate data flow, and the 'through the box' arrow indicates changing the properties of the box. (A bit like the wiper arrow in a variable resistor or capacitor, or a thermistor (NTC).)

This can of course be seen as just another type of data input, but it is often used to indicate an input that has more 'fundamental' effect, like on-the-fly changing ofPID paramaters, or updating a neural network.