This touches on a concept that we call "Attribution" in Archera which handles which RIs & SPs are applied to which resources within your account, along with associated costs & savings.
AWS has a default attribution strategy, where all commitments are applied against all resources based on what gets the highest discount, and for resources that get the same discount, attribution in a arbitrary manner. This behavior applies both to commitments purchased natively through AWS costs explorer and commitments purchased through Archera, even if they were purchased for a particular segment.
By default Archera simply uses the AWS default attribution strategy and the reason is that, that attribution strategy is what is going to be reflected on your AWS bill and your usage report. Using a different attribution strategy would result in AWS and Archera disagreeing on the exact cost of each resource.
However, for customers who want to have a different chargeback model than the AWS defautl, with a custom attribution strategy, we can support custom attributions of any RI or SP via our Custom Reporting Functionality which is used by our enterprise customers to perform analysis on COGS, margin, BU showback/chargeback and other custom functionality.