On the Embraer 170, the F seating is 1(A)+2(DF) and the E seating is 2(AC)+2(DF). So 3A (window) can use 2A (window) for underseat storage, but it is true that 3C (aisle) has none (being that the next thing in front of 3C is the cockpit door a couple meters forward). 3D (aisle) can use 2D (aisle) for underseat storage and 3F (window) can use 2F (window) for same.