The eighteen Armanen runes are Guido von List's, published in 1908, and every one of them is a historical character with his own naming and meaning laid over it.
The character behind it
Ar corresponds to the Younger Futhark character whose Old Norse name was ar, meaning year in the specific sense of a good season and a successful harvest. The rune poems treat it as plenty, and the Old English tradition glosses its equivalent in the same direction. It is one of the more favourable characters in the medieval material by every surviving account.
What von List did with it
He kept the A value and the name, and assigned the character a meaning within his own eighteen. Because the historical gloss is uncomplicated and consistent, this is another of the runes where his row stays close to the medieval material rather than departing from it.
The frame
Von List described the eighteen as an ancient Germanic priestly alphabet recovered from the past, with the count matched to the eighteen rune-charms of the Havamal. Runologists reject the recovery claim: no attested futhark has eighteen characters, and the Havamal does not describe an alphabet.
Reception
The row was taken up in volkisch circles and, during the 1930s and 1940s, by National Socialist organisations. The atlas records that reception as documented history and does not present the Armanen row as Germanic antiquity.