Hi ihaveeyes,
As you know, Purview is all about data protection. Compliance Manager is one of those functions for data protection.
Compliance Manager will generate a data map of all of your data and let you set policies to ensure your data is being used they way it should.
Yes you can think of it as a sub-functionality, as what you want to do with your data from a security perspective can take many roles.
Compliance Manager will normally be tenant focused (from my experience), but you can have it focus per subscription (see link below).
References:
use-microsoft-purview-governance-portal
classification-insights
concept-best-practices-network