Hello Drolma, all,
Short extract from "A Survey of Paramattha Dhammas" by K. Sujin Borkharnmanaket (transcribed by Nina van Gorkom) pp37 & 38
"Nibbana paramattha dhamma can be classified according to three characteristics:
Nibbana is called 'voidness' (su~n~natta), because it is devoid of all conditioned realities (sankhara dhammas). It is called 'signlessness' (animitta) because it is void of signs, or characteristics, of conditioned realities. It is called 'desirelessness' (appanihita) because it is without any basis of desire, namely, it is not condtioned reality.
When someone has developed panna to the degree that he is about to attain enlightenment, he may penetrate the dhammas that appear at those moments as impermanent, as dukkha, or as anatta. Only one of these three general characteristics can be realised at a time. When he attains nibbana his way of emancipation is different depending on which of the three general charactgeristics of conditioned dhammas he has realised in the process during which enlightenment is attained. When he realises dhammas that appear as impermanent he becomes liberated (realises the Four Noble Truths) by the emancipation of signlessness (animatta vimokkha)  When he realises dhammas as dukkha he becomes liberated y the emancipation of desirelessness (appanihita vimokkha)  When he realises dhammas as anatta (non-self) he becomes liberated byu the emancipation of voidness (su~n~natta vimokkha). 
 Vimokkha means 'liberation, emancipation'.
 Dhammas that arise and fall away are not happiness, they are not worthclinging to, they are dukkha. The person who has realised dukkha when he is about to attain nibbana becomes emancipated by desirelessness.
 Dhammas are void of the self."
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