The Unicode Technical Committee is seeking feedback on a proposal to define a new character property for the class of prepended concatenation marks, also referred to as prefixed format control characters or, more generically, as subtending marks. Characters in that class include U+0600 ARABIC NUMBER SIGN and U+06DD ARABIC END OF AYAH. The new property, named Prepended_Concatenation_Mark and targeted for Unicode 9.0, would provide a mechanism to handle subtending marks collectively via properties rather than by hardcoded enumeration. A detailed description of the issue and how to provide feedback are given in Public Review Issue #310.