The UniCharArrayOffset type is the edge offset in backing store memory that corresponds to the beginning of the range of text within a text layout object to be operated on. Functions that perform layout operations on text do so within this range of text, not the entire text buffer. If you want the range of text to start at the beginning of the text buffer, you should pass the constant kATSUFromTextBeginning, described in Text Offset Constant.
typedef UInt32 UniCharArrayOffset;
