This is BA’s wording which describes it quite well

”The length of the seat is 6ft (182.9cm), but the seat also reclines into what is called the “Z” position, which extends the seat to 6ft 6 inches (198cm). This position supports the legs and lower back in a similar way as a sun lounger”

Setting the seat into the Z position is quite simple. Pivot the footstool upward so it is vertical, then gently push it downward. Once it is as low as it will go, pivot the stool so it rests at an angle, then recline the seat. It’s very comfortable and a feature I miss with the new seats.