<NO>OOXML has a dedicated page about the Binary Space in the file format, which originally allowed any user to put any kind of data in there, and especially binaries. One dominant vendor especially like to put binaries in there, in order to foster innovative patented binary extensions:
This binary part supports the storage of arbitrary user-defined data.
<Relationships xmlns="…">
<Relationship Id="rId7"
Type="http://.../customProperty" Target="CustomProperty.bin"/>
</Relationships>
The problem has been mentioned by several delegations in their comments, such as CA-0009, CL-0042, CL-0047, CL-0052, DE-0007, DK-0102, FR-0015, GB-0042.
Now, their ECMA slave is trying to decorate the Binary Space with some XML container:
Agreed; the following changes will be made to Part 1, §12.3.5, page 68, lines 2432:
12.3.5 Custom Property Part
Content Type: Any content, support for which is application-defined. [Note: Some examples of formats which might be supported include: application/vnd.openxmlformats-officedocument.spreadsheetml.customProperty application/xml end note]
[Note: It is recommended that a Custom Property Part contain XML content for improved interoperability; however, there is no requirement on the format of the content contained in a Custom Property Part. end note]
Conclusion: Since there is no requirement on the format of the content, any vendor can put its proprietary binary extensions in there.
Good Bye Interoperability!