Arabic diacritics

[عبد الرحمان أيمن]

hi,

regarding the Arabic diacritics. e.g. for the Shadda, we
have:

1. The form that people type:
http://unicode-table.com/en/0651/

2. An Isolated form. It should be the same, but looks different in the
Unicode table, which is confusing me now.
http://unicode-table.com/en/FE7C/

3. A medial form:
http://unicode-table.com/en/FE7D/

When do I use 1/2, and when do I use 3?

some diacritics has e.g. isolated and medial forms. Some have
only one of these forms, some have both. So, where does each of them go?

respectfully
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