Nairan is to check and read any document for the Emperor to read, or for Emperor to make decision about, or the position of Ryoge no Kan (a position which is not included in the regulations).
There was an imperial letter for Nairan for the Regent and the Chancellor, sometimes a position like Sadaijin (the Minister of the Left), Udaijin (the Minister of the Right), Naidaijin (the Minister of the Interior) or Nagon (senior official of the Imperial Japanese Council of State) would received the same imperial letter, Nairan.
There was one example when Nairan was ordered to a position except for Regent or Chancellor, when Emperor Daigo was in power, instead of having those two positions, FUJIWARA no Tokihira and SUGAWARA no Michizane became Nairan. There were no positions for regent or chancellor set up during Emperor Daigo's era, and FUJIWARA no Tokihira and FUJIWARA no Michizane became Nairan.
When the Regent, FUJIWARA no Koretada died while Emperor Reizei was in power, Koretada's younger brother, FUJIWARA no Kanemichi, was to become Chancellor, but he was still in the position of Chunagon (vice-councilor of state) and this did not satisfy the requirements to become Chancellor, once he was appointed to Nairan, Naidaijin (the Minister of the Interior), he was then appointed to the Chancellor, after having the Naidaijin rank.
While Emperor Ichijo was in power, FUJIWARA no Michitaka was in serious condition, his oldest son, FUJIWARA no Korechika became Nairan for a limited period until Michitaka could recover. However Michitaka died and Korechika's position of Nairan was cancelled.
After that Michitaka's successor, his younger brother FUJIWARA no Michikane, also died, and there was a struggle between another younger brother, FUJIWARA no Michinaga, and Korechika to become successor for the position of Chancellor, but Emperor Ichijo appointed Michinaga as Nairan and did not make any further official decisions. Michinaga did not wish to be appointed Chancellor in order to maintain his political base, as being the Grand Council of State, he was in the highest political position of 'Nairan Sadaijin (Minister of the left) for twenty years, until his grandchild, Emperor Goichijo appointed Michinaga as the Chancellor, during this time there was no one in fact in the position Chancellor.
While Emperor Konoe was in power, Chancellor FUJIWARA no Tadamichi incurred the anger of his father, FUJIWARA no Tadazane, and Tadamichi's position as leader of the Fujiwara clan was passed to his younger brother, FUJIWARA no Yorinaga; at this time, Tadazane asked the Emperor to strip Tadamichi of his position as Kanpaku (chief adviser to the Emperor), but Emperor Toba, who was ruling as a cloistered emperor at the time, helped resolve the situation by issuing an Imperial decree for Yorinaga to become Nairan in exchange for Tadamichi being allowed to retain his position as Kanpaku. (Until this time, it had not been possible for the positions of Kanpaku and Nairan to be filled at the same time.)
However the Cloistered Emperor Toba was not comfortable with Tadazane getting involved in personnel matters, and after the sudden death of Emperor Konoe, he stripped Yorinaga of his position as Nairan, an action that would later result in the Hogen War.
Tsunetada KONOE was the Chancellor for Emperor Komyo of the Northern Court, during the period of the Northern and Southern Courts, he ran way and joined the Yoshino Imperial Court. Emperor Godaigo had his policy not to have the regent and the chancellor, he appreciated the Tsunetada's royalty to him and appointed him as Nairan on this occasion.
Nairan in general
Referring to the above, it is called Nairan or Nairankai to show the displayed products beforehand to certain people before they are officially opened to the public.
It was also called Nairankai to have a display for certain targeted clients.