The multi-tenancy feature of Orchard Core is great: A tenant is basically a subsite with its own independent content and configuration, under its own domain or URL prefix. You can use tenants to e.g. host websites for multiple customers of yours from a single Orchard Core app. The sites won't know anything about each other but they'll run from the same app built from the same codebase, and have access to the same modules and themes. This makes maintaining such sites very efficient, both for hosting and for development.
What if you want tenants to be not that isolated though? What if there is certain content or configuration that you actually want to share among tenants or some functionality that you want to centralize on one tenant? You can use the APIs we show below to cross tenant boundaries and use any service from another tenant!