I have noticed that the term ‘Witness’ is often used interchangeably with the terms Atta and Supreme Self (i.e. that which is not one of the five aggregates). I do not think ‘Witness’ is a good term. Using this term to instruct others and yourself before or during meditation can subconsciously or consciously privilege your visual experiences – as if you should be looking for something, as if the Supreme Self is something that can be seen with either the outer or the inner eye.

The Supreme Self can no more be seen than it can be tasted or smelled.
Can you imagine telling your Zen Master that you have tasted the Eternal Self– and that it tasted like chicken soup?
That is ridiculous. (Actually on some level that might be true, but never mind about that right now.)
That which is "prior" to the aggregates (i.e. the True Mind, the Supreme Self, the Atta) is not a sensation; it is not a sound; it is not a taste or scent; it is not a visual experience.
The term ‘Witness’ is just as inappropriate as the term ‘Smeller.’
It is just wrong!
Looking, listening, and feeling for the Supreme Self is an important exercise. It is to play the ‘that-is-not-the-Self’ game (i.e. the doctrine of anatta game). The purpose of this game is to learn what you are not. How does the game work? You look for some experience, some candidate that could be the Self, you see that the experience is transient, that it is empty of independent existence, and you throw it away (remembering to embrace it later) saying: ‘That is not the Self.” Eventually you learn to stop looking with your eyes (the Self is not infinite empty space) and you learn to stop listening with your ears (the Self is not blissful silence of the mind). One day you will realize that you have thrown everything away – that you have declared: “That is not the Self,” for the last time.
On that day, your search for the Supreme Self will have ended.
You do understand, don’t you, that you never actually see The Supreme Self, you never actually say: “This is the Self”?
You simply realize that you are "prior" to the aggregates – that you are "prior" to all that is not the Self.
You simply realize that you are free.
At least, I think that’s how it works, what do you think?
Tallis