As told by a Customer Support Crewmember:
I had a Customer call because she wanted to make sure that her husband’s bad knee was in the aisle for their trip to Jamaica. I advised her to book him an aisle seat on the left-hand side of the plane, any C seat. She then said on the return flight she would book a D seat on the right -hand side of the plane. As I was careful not to chuckle, I advised her to always book a C seat because the front of the plane always goes the same direction even when it travels back to the United States.