Some Hebrew characters (kaf, mem, nun, peh, tsadi) have different forms when appearing at the end of a word (label). The Greek sigma is another example. In Unicode, this is handled by having separate codepoints for the final forms. This is in contrast to e.g. Arabic, where there are much more contextual forms, and shaping is handled on display, and there is only one codepoint per character.
--Paul Hoffman, Director --Internet Mail Consortium