This slide provides another example of context-sensitive positioning of combining characters.

The short vowel ‘i’ in Arabic is usually drawn below the base character. This is normally the only way of distinguishing it from the short vowel ‘a’, which is displayed above the base character.

In this example, however, an additional shadda diacritic is introduced. The shadda is used to lengthen the consonant it is attached to. In that context it is common (though not mandatory) for the ‘i’ vowel diacritic to appear above the base character, but below the shadda so you can still tell it apart from ‘a’.

Note also that this example introduces the idea that you can have more than one combining character associated with a base character.