The difference comes down to where the leg finishes. Full-length leggings extend to the foot. Ankle-length leggings finish at or above the ankle bone.
That small shift changes the feel completely. Less fabric around the ankle. A lighter feel through training. More airflow on warmer days.
Some call them ankle-length leggings. Others know them as ankle grazer leggings or ankle biter leggings. Different names, same idea: a legging that finishes at or just above the ankle.