The Armanen row was published by Guido von List in 1908. This character carries a documented twentieth-century usage that belongs in any description of it.
The character behind it
Yr corresponds to the Younger Futhark character whose Old Norse name was yr, the yew, a tree associated in the rune poems with the bow and with winter. The Old English tradition has related characters glossed in the same direction.
What von List did with it
He kept the name and gave the character the value Y, and read it as the inverted counterpart of his Man rune. Where Man is the human being upright, Yr in his system is the same form turned over, and he reads the pair as the two ends of a life.
The afterlife
In National Socialist Germany the upright character was used in printed announcements and records to mark a birth and this inverted form to mark a death. The convention rests on von List's pairing, and it is documented in the printed material of the period.
That usage is the reason the character is still recognised in contexts far from runology, and the atlas states it rather than leaving a reader to encounter it without explanation. It says nothing about the medieval yew-rune, which is attested long before and carries no such role.
The Armanen row as a whole was adopted in volkisch circles and by National Socialist organisations in the 1930s and 1940s.