"Additional legroom" refers to having more space for your legs to stretch out or move comfortably, usually in a seating arrangement found in vehicles or public spaces. Full definition
In fact, even the third row is far from uncomfortable, though third row legroom has been compromised slightly in the interest of additional legroom in the second row. (rediff.com)
These are our widest seats, which feature additional legroom and no middle seat. (spirit.com)
This seat has additional legroom due to the Exit Row. (seatguru.com)