Two kinds of Nibbāna
(1) Saupadisesa Nibbāna – experienced when the Arahat is in the existential mode.
(2) Anupadisesa Nibbāna – experienced when the Arahat is in the experiential mode.
Although this is so, today the Theravada tradition explains these two kinds of Nibbāna in a different way. They explain it as follows:
(1) Saupadisesa Nibbāna – experienced when the Arahat is alive.
(2) Anupadisesa Nibbāna – experienced after the death of the Arahat.
This point is clarified in a Nibbānadhātu sutta #44 in the Itivuttaka:
Monks, there are two modes of Nibbāna (Dve'me bhikkhave nibbāna dhātuyo).
What are the two? (Katamā dve).
The personal mode, and the impersonal mode (saupādisesa ca anupādisesa ca nibbāna dhātu).
What monks is the personal mode of Nibbāna? (Katamā ca bhikkhave saupādisesā nibbāna dhātu). In this case monks, a monk is an emancipator, free of influences, has actualized the potential, accomplished the task, laid down the burden, realized the ideal, broken the bonds to being, liberated through tranquility and insight (Idha bhikkhave bhikkhu arahaη hoti, khīnāsavo, vusitavā, katakaranīyo, ohitabhāro, anuppattasadattho, parikkhina bhavasaññojano, sammadañña vimutti).
While in this mode, the five senses are active, and able to experience the pleasant and unpleasant sensations, and therefore experiences pleasure and pain (Tassa tiṭṭhanteva pañcindriyāni, yesa avighātattā, manāpa amanāpa paccanubhoti, sukhadukkhaη paṭisaηvedeti).
Yet he is free from lust, hate, and delusion (Tassa yo rāgakkhayo, dosakkhayo, mohakkhayo).
This, monks, is the personal mode of Nibbāna (Ayaṃ vuccati bhikkhave saupādisesa nibbāna dhātu).
What, monks, is the impersonal mode of Nibbāna? (Katamā ca bhikkhave anupādisesa nibbānadhātu).
In this case monks, a monk is an emancipator, free of influences, has actualized the potential, accomplished the task, laid down the burden, realized the ideal, broken the bonds to being, liberated through tranquility and insight (Idha bhikkhave bhikkhu arahaη hoti khīnāsavovusitavā katakaranīyo ohitabhāro anuppattasadattho parikkhinabhavasaññojano sammadaññā vumutto).
In this very life, monks, he remains experiencing no sensations, insentient (Tassaidheva bhikkhave sabbavedayitāni anabhinanditāni sīti bhavissanti).
This monks is the impersonal mode of Nibbāna (Ayaη vuccati bhikkhave anupādisesā nibbānadhātu).
These two did the seer expound ("Dve imā cakkhumatā pakāsitā).
The unconditioned state of Nibbāna (Nibbānadhātu anissitena tādinā).
One is the state to be experienced here and now (Ekā hi dhātu idha diṭṭhadhammikā).
With personality but existence ceased (Saupādisesā bhavanettisankhayā).
Other without personality and without existence (Anupādisesā pana samparāyikā).
Where existence of every kind ceases (Yamhi nirujjhanti bhavāni sabbaso).
The experience of apperception without construction (Ye etadaññāya padaη asankhataη).
The emancipated mind has ceased to exist (Vimuttacittā bhavanettisankhayā).
Has realized the embodiment of Dhamma (te dhammasārādhigamā khaye ratā).
Brought all states of existence to an end (Pahaηsu te sabbabhavāni tādino’ti).
Thus did the Lord proclaim (Ayampi thovutto bhagavatā).
This means Anupadisesa Nibbāna is not the experience of the Arahat after death of the physical body but the Nirodha Samāpatti.