The cushions are sewn into a fixed seat sofa and are therefore unable to be removed. The fabric is secured to the frame underneath the foam, coming up, under the back and out over the top to be attached at the front of the sofa.
Pieces of furniture with a fixed seat require extra fabric to allow the cushions to compress as people sit on it; this ensures there is enough ‘give’ and reduces the stress in fixed seams which can result in seam slippage or the fabric tearing. This extra fabric moves with you to accommodate the compression of the foam, however, if the fabric doesn't move back fast enough or becomes trapped it is vulnerable to sagging or rippling.
A fixed seat needs to be ‘dressed’ after being sat on. Dressing requires smoothing out the fabric on the seat by brushing it back towards the rear. If this isn’t done often enough it can lead to permanent creases on the seat, which isn’t considered a fabric or manufacturing issue but instead the result of insufficient care and maintenance.
Read more about dressing your sofa in our article Common causes of sagging sofa cushions.