Interfaces are contracts between caller and provider. James Stanier, the author, is using interfaces as contracts in order to make expectations explicit while delegating out the provider's implementation.<p>He does not address the contract that you as a manager fulfill as a provider for your reports. He should. Given the volume of material on his site, I'd expect he addresses how managers serve elsewhere.<p>Stanier is talking about managers managing other managers, so the units of implementation are people who orchestrating other people's work and who are serving the needs of their teams in turn. A manager of managers has an interest in "what", the contract, and as well as "how well", the return, and following that, with exception cases, the causal question of "why". As a manager of managers you need to know enough specifics to quickly understand and add value in situations where your chief service is exception handling and diagnostics.