The Shadda character is used for representing geminate (long) consonants. Nukta is used to change the phonetic value of a letter so that it can represent non-native (primarily Arabic) sounds. For example, ja + nukta represents [za]. Both Shadda and Nukta are written above the consonant to which they apply, in which space certain vowel diacritics may also be written. (Nukta may also be written above a vowel.) In the case where more than one sign needs to be written above a consonant, complex positioning must be applied as follows.
Vowel sign e or ai with shadda -> shadda is written to the right of the vowel sign.
Nukta with shadda -> shadda is written below nukta.
Vowel sign e or ai with nukta -> nukta is written to the right of the vowel sign.