Which features are shared and which are separated between the accounts?

Groups: Groups that are set up for sub-accounts are the only groups he can work on, and these are shared with the master account and any other sub-account assigned with the group/s.

Tags: Tags are separated. Sub-account users can set their own tags. Sub-account tags are not shown on the master account.

Reports: Reports are separated (unless its a user with Admin permissions). Reports that a sub-account has set up are not shown on the master account (unless its a user with Admin permissions).

Saved Filter: Saved filters are separated. Saved filters that a sub-account has set are not shown on the master account.

Custom templates: Sub-account users can use custom templates if the master account holder has made it applicable.
The sub-account user can also set new custom templates, which are not shown on the master account.