Unsatble's explaination sounds good.

I would like to add one thing. Crystals are peices of quartz. A peice of quartz will vibrate at a given frequency. Not all peices of quartz are identical. It is best to have the TX and RX crystals made from the same peice of quartz. They are generally considered paired. I said it is"best"if they come from the same quartz. However, you will see people flying with an unmatched set of crystals.