Unicode combining characters
Unicode code points that attach to the character before them instead of standing alone — the mechanism behind accents, and behind glitch/zalgo text.
Combining characters are Unicode code points designed to modify the character immediately before them rather than occupy their own visual space. The everyday example is an accent: the é in "café" can be encoded as a plain "e" followed by a combining acute accent mark, which renders on top of the e instead of next to it.
Unicode groups these marks by where they render relative to the base character — above, through (overlay/strikethrough-style), or below. Ordinary text uses at most one or two combining marks per letter. Nothing in the standard caps how many can be stacked on a single character, which is the entire mechanism behind glitch and zalgo text: enough marks stacked together and the base letter becomes visually buried.
This site's glitch font generator exposes those three zones — above, through, below — as independent sliders, rather than one combined intensity setting, because they behave differently: "through" marks are the ones most likely to render as a box on older systems.