When the head is correctly positioned, breathing and swallowing should be effortless. If head support is needed, aim to keep the neck straight, the chin slightly elevated and an eye-line that encourages interaction with the environment. Fixing the head in one position can be frustrating for the person, so consider the shape and firmness of any support, and bear in mind sightlines and hearing.
Back support is essential for comfort and upper trunk function. Any gaps between the user and the chair back mean they will have to work harder to maintain their position. Their weight will also be concentrated onto small contact areas, increasing pressure. Check for gaps behind the pelvis, in the lumbar area, under the shoulder blades and below the neck. You can profile the chair back to help fill the gaps, using headrests and lateral supports to achieve full contact.
The shape of a chair should never restrict whatever arm movement the individual has. It is not comfortable for somebody to use their arms to control their trunk position for long periods of time - this quickly makes the neck and shoulders feel stiff and sore. Arms should be supported on armrests without pushing up the shoulders. Trays and tables should be at the right height and position to reach and grip easily.
The position of the thighs helps to stabilise pelvic position and affects circulation to the lower legs. Aim to keep the knees at the same height as the hips so that the soft tissues on the underside of the thighs don't become overly compressed. If the knees are positioned apart, the sitting position will be more stable from side to side.
Poor circulation and lack of movement means that extra care needs to be taken when thinking about support for the lower legs. A lot of upper trunk functionality depends on having foot support, so that the user can control their balance and extend their legs helping keep their pelvis in the right position. Support the calf fully, but avoid pressure on the heels.