An alpine touring boot is either compatible with a standard alpine ski binding (DIN normalized), a Tech binding, or both. Some boots, however, can be fitted with removable sole blocks to make them more versatile (sold with boots or available separately).
A stiff boot will have a high flex rating (120-130+), while a softer boot will have a lower flex rating (100-110). Stiffness benefits you during the descent, but it might cause you pain on the skin track-consider whether you prefer superior comfort or performance. A boot's flexibility on the walk or skin up will also be affected by the range of motion on the walk/ride hinge.
A carbon cuff or tongue, lightweight plastic shell, minimalist buckle design, or honeycomb structure help reduce the weight of an AT boot so you can move faster and feel less fatigued during a long tour. However, if downhill performance is more of a concern, you might want to consider a heavier boot.