Demialterous can be a orientation on its own or can combined with other orientations. For example, one could be demialterous and gay, meaning that when one does experience alterous attraction it's only towards people of the same gender.
The term appears to have been first referenced/used by Pride-Flags on DeviantArt on July 5th, 2015, however it is unknown if this is the original coiner.
The first usage online appears to have been by by DeviantArt user GodofDarness18 on October 7, 2013, in their Pixel Sprite Montage.
The flag was made by Pride-Flags on DeviantArt on July 5, 2015. The light blue represents alterous attraction, while the right facing black triangle is associated with any kind of demi-attraction.