>>216534504
I can't say for certain, but judging from sample pages online, the X mark likely indicates non-jouyou kanji as the Algeirianon said >>216534810
. The triangle mark seems to denote kanji not taught in elementary school, and kanji taught in elementary school seem to have corresponding grade levels added.
For example, a word "揚羽蝶" (あげ-は-ちょう= swallowtailed butterfly) has "Δ2X" marks. 揚 is actually taught in middle high school and 蝶 is non-jouyou.