Markup Compatibility Namespace for Package Digital Signatures

The package implementer shall not use the Markup Compatibility namespace, as specified in Annex F, “Standard Namespaces and Content Types,” within the package-specific <Object> element. The package implementer shall consider the use of the Markup Compatibility namespace within the package-specific <Object> element to be an error. [M6.32]

Format designers might specify an application-specific package part format that allows for the embedding of versioned or extended content that might not be fully understood by all present and future implementations. Producers might create such embedded versioned or extended content and consumers might encounter such content. [O6.12]
example: : An XML package part format might rely on Markup Compatibility elements and attributes to embed such versioned or extended content.

If an application allows for a single part to contain information that might not be fully understood by all implementations, then the format designer shall carefully design the signing and verification policies to account for the possibility of different implementations being used for each action in the sequence of content creation, content signing, and signature verification. Producers and consumers shall account for this possibility in their signing and verification processing. [M6.33]