Assume I was using the platform fully across all domains. I'm managing the engineering item definition, the manufactured item defintion, SBOM, e.g. 

I'm looking for best practices and reference information on how to use this characteristic and how it propagates across the platform.

Which objects should be assigned an enterprise item number?

Can I assign an EIN to an object in any of these domains, or should EIN only be assigned to Physical Products and have manufacturing and service parts inherit an EIN from them?

Should I refrain from assigning EIN to Physical Products, because they're not "real" yet and only assign EIN to manufactured/procured parts and service parts?

If I do assign EIN to a physical product, how does that characteristic propagate to manufactured/procured/service items that implement that physical product? 

How should EIN work with alternates/substitutes? (if I have a physical product that is an alternate for an item assigned an EIN, say, because it's a different installed geometry vs. the nominal purchased shape) 

Thanks for any info or reference documentation that can be shared.