Hi Candice00,
In the 8 Mansions theory, sitting position is used to describe the type of house you are staying. For example if your house is facing East your sitting position will be West and since West fall under Dui Trigram, your house will be known as Dui House. Dui is a metal element and if you are West group person then this house is suitable for you.
Furthermore if your Day Master is a weak Water then Metal is also your favorable element.
Bernard