AirAsia and AirAsia X pick a seat is another way of AirAsia to charge money from you which it was previously offer to you FREE, because you can request it during check in, however this has been changed long long time ago, which I also can’t recalled how long it was.
Pick a seat is an option that let you choose your desire seat during your journey with AirAsia and AirAsia X. You can choose windows seats, hot seats, or together with your loves one.
Watch the youtube video below which is prepared by AirAsia and get some concept about it, if you still out of idea.
Basically, AirAsia uses Airbus A320 for their domestic and ASEAN flights while AirAsia X uses Airbus A330 and Airbus A340 for their long haul flights and the charges for pick a seat of course are different among AirAsia and AirAsia X.