Only the Futaba 72mhz recievers come tuned Hi or Low band they recomend When you change frequency you keep in either Hi or low otherwise send in the Reciver to be fine tuned for maximum range. The others RXs including the 75mhz RXs are center band tuned and will take any crystal, They now it's for ground use and radio range isn't as critical and in an airplane

Example, If you have a 72mhz on channel 20 (low band) you don't want to change it to channel 50 (hi band),, You'd want to chose a channel 11-35,,,high band channels are 36-60

But as far as I know Futaba is the only one that recomends this,,72mhz Hitec recievers come in only one form and will take any crystal in the 11-60 range

Been an Airplane guy for 30 Years