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  • regis hattori thinks this is interesting:

However, we must keep in mind that this does not place the referenced Aggregate inside the consistency boundary of the one referencing it. The reference does not cause the formation of just one whole Aggregate. There are still two (or more), as shown in Figure 10.5.

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Cover of Implementing Domain-Driven Design

Note

I thought that in this case we had 2 aggregates: one formed by BacklogItem and another formed by BacklogItem + Product.