Prescaling allows the counter to count a signal that is faster than the maximum timebase of the counter. The TIO counters offer 8X and 2X prescaling on each counter. You can disable prescaling. Each prescaler consists of a small, simple counter that counts to eight (or two) and rolls over. This counter is specifically designed for this application and can count signals that are faster than the general purpose counters. The CtrN source signal on the general purpose counter is the divided signal from the simple counter.
Prescaling is for two-counter period and frequency measurements in which the measurement is made on a continuous, repetitive signal. The prescaling counter cannot be read; therefore, you cannot determine how many edges have occurred since the previous roll-over. Prescaling can be also used for counting edges provided it is acceptable to have an error of up to seven counts when using 8X prescaling or one count when using 2X prescaling.