Puppet is an extensible automation framework, a tool, and a language. We can do great things with it and we can do them in many different ways. Besides the technicalities of learning the basics of its DSL, one of the biggest challenges for new and not-so-new users of Puppet is how to organize code and put things together in a manageable and appropriate way.
It's hard to find comprehensive documentation on how to use public code (modules), custom manifests and custom data, where to place our logic, how to maintain and scale it, and, generally, how to manage, safely and effectively, the resources that we want in our nodes and the data that defines them.
There's not really a single answer that fits all cases; there are best practices and recommendations, but ultimately it all depends on our own needs and infrastructure, which vary according to multiple factors. One of these principal factors is the characteristics of the infrastructure to manage itself...