Unicode document.  UTC response in 01-404 says:

Arabic Enclosing Marks

[89-C27] Consensus: For the ARABIC END OF AYAH the UTC prefers the head model rather than the tail model, like the Syriac abbreviation mark. [L2/01-428]

[89-C28] Consensus: It should be made clear in the standard that the graphical display of combining enclosing marks apply to preceding grapheme clusters. [L2/01-428]