Correct, with a 4 pin fan the additional pin is a pulse width modulation signal. 3 pin would have Common, +12V, and a Tach signal, while 2 pin would be +12V and Common only.
On your particular board, the 0.75 and 2.5 are (somewhat) arbitrary values for something Gigabyte refers to as "PWM Value/ degree C". Depending on the hardware, there are so many "steps" to the PWM control, (eg. 0 - 127), and each degree C that the cpu temperature increases, that value will bump up by 2.5 steps until at maximum speed. It used to be that you could also set the starting PWM Value, which I believe they have removed that from some of their boards, which doesn't help users unfamiliar with those boards to understand it's functionality.
However, some boards have the functionality of PWM control on the +12v signal, which negates the need to utilize a 4pin fan for RPM control.