When considering how to organize your specification into a set , there are no formal rules. Domains are fuzzy concepts on purpose. However, here are a few things that might help:
- Domain, Subdomain, Bounded Context, Problem/Solution Space in DDD: Clearly Defined
- Domain Drive Design: Domain Definition
- Use the communication structure of the organization per Conway’s Law
- Use the Event Storming process, and then look for the event groupings. Isolated groups are typically subdomains.