Good question, if I'm honest I never really gave much effort over to learning how PAPI and VASI lights actually function, only what they are supposed to do. Someone else actually gave me the numbers for the lights at Ascension, and my runways only use PAPI, not VASI.
As far as I understand it, the first 2 numbers represent how the AOA will affect when the lights change colour, the last number is the distance of the lights from the end of the runway.
So for eg, the main landing runway at Ascension is PAPI 5 3 530