This one is fairly new to me. Character units (CH) are fairly niche and typically used to set the width of paragraphs to help improve legibility and create content that is comfortable to consume and easy to scan.
A single character unit
width: 1ch should result in a paragraph where every character is placed on a new line– and it does. Although in reality, the
ch unit isn’t really counting characters, but instead estimating the number of characters per line, by taking the width of the zero character in any given typeface and multiplying it by the value