In Syriac, words are sometimes written in an abbreviated form, omitting some of the last letters.
In such cases, a special overline is drawn over some of the final letters of the abbreviated word.
To implement this special overline in Unicode, a special control character was conceived: the Syriac Abbreviation Mark (or SAM), described in section 9.3 of the Unicode Standard.
[1] It is inserted in the stream of characters just before the first letter which should be marked with the overline (visually, to the right of it, since Syriac is a right-to-left script).
OpenType also has stch “feature tag” to pave the way of proper rendering.