Hey have a look a little bit further up in the same table. That part of the table is only if you have an actual oscillator for Timer 1.
If you have an actual clock source (frequency input) then you are interested in the
T1CKI input period.
For example lets say you have a standard 16F877a (rather than low voltage) and are using the counter in Synchronous mode with a prescale value of 4. Your minimum input period would be the greater of 30 nanoseconds or (TCY+40)/(Prescale Value). If you are running the chip at 20MHz, then your TCY = 200ns.
This Part is clear to me and thanks to you
The equation then becomes (200+40)/4 = 60ns. 60ns input cycle time is equal to (1s/60ns) 16.667MHz maximum input freq.
this part also clear and thanks
HTH.