Keep Local BOMs and Global BOMs in Separate Business Data Projects

Keep local BOMs and global BOMs in separate Business Data projects, unless you have a compelling reason to keep them together.

This is particularly important for application upgrade, as local and global BOMs have different compatibility requirements:

Local BOMs: You can still upgrade a Business Data project if a local BOM contains incompatible changes, but doing so could result in failure to migrate a dependent process instance to the upgraded version. See Process Migration.
Global BOMs: You cannot upgrade a Business Data project if a global BOM contains incompatible (that is, destructive) changes. See "Upgrading a Case Data Model" in the Case Data User's Guide.