2.1.11 Part 1 Section 12.3.5, Custom Property Part

a.   The standard says that a package shall contain at most one Custom Property part, and that part shall be the target of an implicit relationship from the Workbook part.

Office permits a package to contain zero or more Custom Property parts. Each such Custom Property part shall be the target of an explicit relationship from the Worksheet part.

This note applies to the following products: 2007, 2007 SP1, 2007 SP2.