Work in progress…
Sub-categories mostly behave as their own categories. They just happen to be nested under a parent category for organization.
Subscribing to/Muting Sub-categories
Currently you must subscribe to - or mute - each category and subcategory individually. Subscribing/muting a parent category doesn’t automatically perform the same action for the sub-categories.
This especially makes sense in a community like Mozilla where each team has more general discussion, but also has very specific technical discussion. Someone may only be interested in the specific discussion on a sub-project, or they may only be interested in the general announcements to follow the broader work of the team.
You can subscribe to and mute categories and sub-categories in your preferences, or on the main page for that category/sub-category.