Affinity Designer has several different colour models, some capable of working at 8 or 16 bit depth.
Your document's bit depth refers to the amount of tonal or colour information that can be stored per pixel. The higher the bit depth, the more tonal information each pixel can store.
In Affinity Designer, you need to choose a colour format (mode) to work in when creating a new document. This sets the colour model and the bit depth (8 or 16 bits) for the document you are working on.