2.5.115 RelID
RelID is an XLNullableWideString (section 2.5.167) structure that specifies a relationship identifier, as specified in [ISO/IEC29500-2:2012], section 9.3. The length of the string MUST NOT exceed 255 characters. The string MUST NOT contain a zero character (0x0000). A string that is NULL means that the relationship is not specified.