what does the symbol for Never Fade represent/stand for?

It's only been a while since reading the book but i already forgot what the symbol on the Never Fade book stands for. The other books in the series explains them. For example, the trident like symbol for "The Darkest Minds" (the first book of the series) was explained to be the PSI symbol, and in "In the Afterlight", the crescent moon was said to be a "Road Code" which the kids could use to communicate to other kids that the area was safe to rest in. Anyways, if anybody remembers, please let me know, i won't be able to sleep tonight until i find out what it means. Thx.