From Rigpa Wiki
The ground of Dzogchen is described as being endowed with three qualities: its essence, its nature and its compassionate energy. The second quality, its nature, is spontaneous presence (Tib. ལྷུན་གྲུབ་, lhündrup; Wyl. lhun grub). This is not like the empty aspect, primordial purity, but the term ‘spontaneously present’ indicates that this is the nature out of which all the phenomena of samsara and nirvana arise, and into which they are all absorbed.
As Longchenpa says in The Wish Fulfilling Treasury, it is the basis for karma, disturbing emotions, and the phenomena of samsara, but not in an entirely dependent way. The adventitious phenomena are not part of the nature of clear light, and therefore it is said that although it provides a basis for them, they are not entirely dependent on it. It is similar to clouds in the sky. In some sense they depend upon the sky, but no matter how thick they are, clouds never become part of the nature of the sky itself. They can be separated. The sky and the clouds are both present, but without touching or becoming part of one another. By contrast, the phenomena of nirvana depend on the clear light nature inseparably, just like the sun and its rays. The kayas and wisdoms have always been part of this nature and are never separate from it. The spontaneously present or perfect nature is the basis out of which all pure and impure phenomena can arise. 
- ↑ His Holiness the Dalai Lama, The Vision of Enlightenment, pages 312,313, published by The Tertön Sogyal Trust, ISBN 0 95 312514 9. Also published by Wisdom Publications as Mind in Comfort & Ease, The Vision of Enlightenment in the Great Perfection. ISBN 0-86171-493-8
- His Holiness the Dalai Lama, The Vision of Enlightenment, pages 314,315, published by The Tertön Sogyal Trust, ISBN 0 95 312514 9. Also published by Wisdom Publications as Mind in Comfort & Ease, The Vision of Enlightenment in the Great Perfection. ISBN 0-86171-493-8
- Holiness the Dalai Lama, Dzogchen : The Heart Essence of the Great Perfection, published by Snow Lion, ISBN 1-55939-156-1
- Sogyal Rinpoche, The Tibetan Book of Living and Dying, published by Ryder. ISBN 0-7126-1569-5.