Tach input controller

can you please give solutions to below queries?
The below statement i got from Parker TRM document.
1. Based on the tachometer input, the period and window length are stored in the tachometer registers. Software reads the data
from these registers periodically, calculates the RPM, and compares with the target RPM.PWM parameters are determined in
the RPM correction logic block and serves as an input to the PWM output controller.

Can you tell me what is RPM correction logic and period of thermal fan est-45 temperature reading? what is HI counter logic in TX2 NX?