- Lots of active users; need to add a second mailbox server.
- Users sending/receiving huge volumes of email; need to add a dedicated MTA server.
- Lots of users sending/receiving huge volumes of email; need to add second MTA server with LDAP replica.
That's why the documentation talks more about what each of the packages covers in terms of functionality, and how you manage the installation scenarios accordingly. There are specific examples in there too.
Basically, it takes some analysis to identify the current scarce resources in your system, consider what the next scarce resource(s) will be, and add a second server with the mix of appropriate packages for your environment.
Put differently, there is no one "right way" to scale a Zimbra system, which is why the simple set of instructions you are seeking aren't there.
Hope that helps,