If your Advance Decision contains a refusal of life-sustaining treatment then it must be witnessed by someone. Your witness can be anyone with capacity over the age of 18.
Your witness should watch you sign your Advance Decision. They should then add their own signature and write their name and address in the space provided. They are witnessing you signing your Advance Decision, and the fact that your signature confirms the wishes you’ve written in it. The witness is not signing to say that you have capacity to make the decisions in the form.