question about integrating with ERP

Summary

Need to understand how ERP and Engagement Cloud integrate and at which level

Content

A customer's plan is to implement Oracle ERP but meanwhile they are capturing information in OEC (Engagement Cloud). They would like to understand how the two integrate and at which level.

As an example, if they capture orders in OEC - which ends up in an invoice that ultimately needs to be tied to an A/R - do they have to be tied to an account in OEC? Is there such a correlation even?  Or can they associate the billable/ A/R info with a customer? Assuming they use projects in ERP and integrate with OEC; is there a relationship between business objects there and the ones in OEC?  Does a project have to be assigned to an Account in OEC, or can they be assigned to Contact (or does it matter