When we say "I AM" we are confirming our being, we are confirming that we are certain and intimate with exactly what is being pointed out in this or any other "nonduality" writing.
We are certain of it, yet it isn't an objective knowledge, not a thing in our experience. The mind doesn't like to be on uncertain ground - it must resolve things in an objective way. Therefore identifications come - I am the body, I am the mind, I am thinking...
So initially, we point allowing for this objective world, pointing out that in this objective world, what you are is not a thing to be found, not an objective experience. What you are is the subject to the world, yet it is also subject to the body, subject to thoughts. You are aware of these.
In other words, you are the subject, yet we can't seem to grasp that AS the subject, you cannot be located. You cannot be found in terms of objects or experiences. You are the subject. What is the nature of the subject?
You can answer this with certainty already, for you are that. To be the subject means to know, to be aware, to be that to which the objective appears, to be that by which the objective is known. Isn't that what the word "subject" means?
So AS the subject you cannot appear to yourself, you cannot know yourself objectively, through experience. Yet you know you ARE - because you can say "I AM" with certainty.
What does this mean? It means that WHAT you are isn't locatable, it isn't a form or shape or object or experience. It's just YOU. You know that you ARE - this means you know you are present, somehow, without being findable as another experience.
You know that you ARE, because KNOWING is happening. It is not true? Knowing is happening - knowing is present - isn't this HOW you know you ARE? Isn't this knowing the subjectivity which is referred with the words "I AM"?
So we can say the "subject" is ALWAYS Awareness. What we "call" awareness.
This is in fact so self-evident it is overlooked. It is overlooked because the mind needs some "thing" to place that beingness upon. A body. A mind.
Yet in actual reality or experience, we know ourselves as the subject of each experience - yet we miss that the subject itself is not findable as another experience - the subject has always been just this presence of knowing - presence-awareness - whatever words you like to use.
When we examine what it means to be the subject-awareness, we find the nature of this "awareness" to be without bounds, without form, without attributes, without limits. We cannot find it, like space, yet it fills everything - it is the basis or ground of all experience. We find our true nature is limitless, like space.
Yet even space you objectify - space is an object of awareness. Space does not know you - you know space. Similarly time does not objectify you, you objectify time - you know time. We find nothing that can objectify this presence of knowing, yet it is the most real, the most undeniable aspect of any experience. All experience is variable, changing, temporary, coming and going. Your presence is stable, invariable, boundless.
Simply notice what is the most stable reality throughout a lifetime of experiences - the always-here-ness, always-aware-ness - you know you are always here, always the "subject" which is itself the knowing of the world, body and mind. You are the limitless activity of knowing.
A man once asked the guru how he defines Truth. The guru said "Truth is that which can never be negated". The man asked "is there such a thing?" The guru said "Yes. It is your Self".
Your Self is that which can never be negated - in a lifetime of variable experiences, what you are has never been absent yet never definable in terms of limits or boundaries.
In other words, you are limitless.