A code unit is the basic fixed-width storage unit of a character encoding form. UTF-8 uses eight-bit code units; UTF-16 uses sixteen-bit code units. A code point can require several code units.[ref]
For example,
U+1F600 uses four code units in UTF-8, F0 9F 98 80, and two in UTF-16, D83D DE00, with values written in hexadecimal. Cutting after one code unit can therefore split an encoded code point. A grapheme cluster can itself contain several code points, adding another reason that storage length and displayed-character count differ. New to topics? Read the docs here!