For instance, there are Rottefella NNN I and NNN II bindings. Alpina and Rossignol make boots for the bindings, but NNN I boots don't fit NNN II bindings, and vice versa. The best way to avoid a disaster purchase is to bring the ski and binding and make sure it fits the boot before purchasing the boot. Next, the style of skiing the boot will be used for determines the type of boot that will be purchased. Classical boots have a flexible sole, while skating boots have a stiff sole and higher ankle support.
Combi boots have a cuff that is removable for classical skiing.
Finally, get a boot that fits comfortably from the get-go. Skiing boots are tough, so they're not going to break in like a hiking boot. If the boot is rubbing a skier the wrong way at the shop or swap, the boot should not be purchased. Ski boots are well insulated, so at most the skier will need two pairs of socks. Remember, a tight boot restricting circulation to the feet can cause numbness just as easily as the cold can.