How to connect AFC terminal on Crystal Oscillator
It's right there in the data sheet on the page you linked: -



Centre voltage is 1.4V and this I take to mean as the voltage (if applied) would do nothing to the factory setting i.e. it stays at the same frequency.

Note that there is a control range of 7 to 16 ppm per volt and this means if you lifted the voltage from 1.4V to 2.4 volts you'll probably offset the frequency by about 11 ppm or 11Hz per MHz - if it's a 50MHz xtal it'll move 550 Hz. Conversely if you took the 1.4V down to 0.4V, the frequency would nominally reduce by 550Hz in 50MHz.

What to do if you don't use it - be prepared to put a potential divider from the supply rail and ground to that pin, roughly generating 1.4Vdc BUT, there's a fair chance it'll work open circuited just fine.