Illiyun (Arabic: عِلِّيِّينَ) This term is the plural form of ‘uluw which basically means 'highness, High places or the persons who sit in high places, or the dwellers of high parts of the mountains'. Illiyyin, is the opposite of Sijjin.
Verily, the Record of Al-Abrar (the righteous believers) is in `Illiyyin, meaning, their final destination is `Illiyyin, which is the opposite of Sijjin.
Illiyin’ is a ledger for the righteous deeds, Allah has written for the Righteous that their position is in the highest stages of skies.
Allah said: No! Indeed, the record of the righteous is in ‘illiyyun. And what can make you know what is ‘illiyyun? It is [their destination recorded in] a register inscribed. Quran Surah Mutaffifeen 83:18-20
Which is witnessed by those brought near [to Allah ]. Indeed, the righteous will be in pleasure. On adorned couches, observing. You will recognize in their faces the radiance of pleasure. They will be given to drink [pure] wine [which was] sealed.Quran Surah Mutaffifeen83:21-25
The above aayaath Allah saw, the people in the ‘illiyun will be brought near to Him and will be given the pleasures of Jannah. 
From Al-Bara' ibn `Azib (may Allah be pleased with him) that he said, We were attending a funeral in Baqi' Al-Gharqad when the Messenger of Allah (peace be upon him) came and sat with us. We remained silent while he (peace be upon him) was digging the grave, until he said, "I seek refuge with Allah from the punishment in the grave." He repeated it three times, and then he said,
"When a believer is about to depart this world and move on to the Hereafter, there comes down to them from heaven angels with white faces like the sun, and they sit around them as far as the eye can see. They bring with them shrouds and perfumes from Jannah (Paradise). Then, the Angel of Death comes and sits by their head, and he says, ‘O good soul! Come forth to forgiveness from Allah and His pleasure.’ Thus, the soul comes out easily like a drop of water from the mouth of a water skin. When he seizes it, they do not leave it in his hand for an instant before they take it and put it in that shroud with that perfume, and there comes from it a fragrance like the finest musk in the world. Then, they ascend to heaven. Whenever they pass by any group of angels, they inquire, ‘Who is this good soul?’ They say, ‘It is so and so, the son of so and so', calling them by the best names by which they were known in the world, until they reach the nearest heaven. They ask for it to be opened to them; it is opened; and (the soul) is welcomed and accompanied to the next heaven by those who are closest to Allah, until they reach the seventh heaven. Allah then says: ‘Record the book of My slave in ‘Illiyyun (in the seventh heaven), and return them to Earth, for from it I created them; to it I will return them; and from it I will bring them forth once again.’
The soul is returned to the body, and two angels come who make the person sit up and they ask them, ‘Who is your Lord?’ They say, ‘Allah.’ They ask, ‘What is your religion?’ They say, ‘My religion is Islam.’ They ask, ‘Who is this man who was sent among you?’ They say, ‘He is the Messenger of Allah.’ They ask, ‘What did you do?’ They say, ‘I read the Book of Allah and I believed in it.’ Then a voice calls out from heaven, ‘My slave has spoken the truth,so prepare for them a bed in Jannah; clothe them from Jannah; and open for them a gate to Jannah.' Then there comes to them some of its fragrance, and the grave is made wide, as far as they can see. Then there comes a man with a handsome face, elegant clothes and a good fragrance, who says, ‘Receive the glad tidings that will bring you joy this day.’ They say, ‘Who are you? Your face is one which brings glad tidings.’ He says, ‘I am your righteous deeds.’ They say, ‘O Lord, hasten the Hour so that I may return to my family and my wealth.’ On the other hand, when a disbeliever is about to depart this world and move to the Hereafter, angels come down to them from heaven with black faces, bringing sackcloth, and they sit around them as far as the eye can see. Then the Angel of Death comes and sits by their head, and he says, ‘O evil soul, come forth to the wrath and anger of Allah.' Then the soul disperses inside the body, and comes out cutting the veins and nerves, like a skewer passing through wet wool. When he seizes it, they do not leave it in his hand for an instant before they take it and put it in that sackcloth, and there comes from it a stench like the foulest stench of a dead body on the face of the earth. Then they ascend to heaven. Whenever they pass by any group of angels, they inquire, ‘Who is this evil soul?’ and they say, ‘It is so and so, the son of so and so', calling them by the worst names by which they were known in the world, until they reach the nearest heaven. They ask for it to be opened to them, but it is not opened.” Then the Messenger of Allah (peace be upon him) recited Allah's saying, ...for them the gates of heaven will not be opened, and they will not enter Paradise until the camel goes through the eye of the needle (which is impossible). Surah Al-A'raf, 7: 40
Allah (Glorified and Exalted be He) then says, 'Record the book of My slave in Sijjin in the lowest earth, and return them to the earth, for from it I created them; to it I will return them; and from it I will bring them forth once again.’ So the soul is cast down.” Then, he (peace be upon him) recited Allah's saying, ...and whoever assigns partners to Allâh, it is as if he had fallen from the sky, and the birds had snatched him, or the wind had thrown him to a far off place. (Surah Al-Hajj, 22: 31) Then the soul is returned to the body, and two angels come who make that person stand and they ask them, ‘Who is your Lord?’ They say, ‘Oh, oh, I don’t know.’ They ask, ‘Who is that man who was sent among you?’ They say, ‘Oh, oh, I don’t know.’ A voice calls out from heaven, ‘Prepare for them a bed in Hellfire and open for them a gate to Hellfire.’ There comes to them some of its heat and hot winds, and the grave is constricted until their ribs are squeezed. Then, there comes to them a man with an ugly face, shabby clothes, and a foul stench, who says, ‘Receive the bad news, for this is the day that you were promised.’ They inquire, ‘Who are you? Your face is one which forebodes evil.’ He says, ‘I am your evil deeds.’ They say, ‘O Lord! Do not let the Hour come.’ Abu Dawud 4753 and Musnad Ahmad Vol.4: pg296 
Some have also said that the term is mentioned in the plural form for emphasis where it means 'highness over highness'. The first commentary in Tafseer Ibn Katheer says that kitab-al-abrar means 'the Record of the Righteous'. They intend to say that the record of their deeds is registered in a ledger, (a complete work), which shows the whole deeds of all believers, and is in a very high and honourable position.
Or, it means that their record is preserved in the highest and the most honourable abode in Heaven or skies over their places, all of which illustrate that their own rank is extremely high. Which would be the contrary of the word sijjeen?
The second commentary says that kitab, here, means 'destiny' and 'the definite end' that Allah has defined for the Righteous in the highest levels of Heaven. Of course, these two commentaries can also both come together, i.e. their Record is registered in a ledger and the ledger resides totally in heaven. Meanwhile, the Divine command governs high over them concerning the position of the Righteous, themselves, and considers them to be in the highest levels of Heaven.