[andrewaylett]

enable/disable is whether the unit's own definitions of where it fits into the dependency tree are used. So even when A is disabled, a Requires dependency from another enabled unit B will cause unit A to start.

Masking removes the unit from consideration altogether, so even if a unit depends on it, it won't start. That means that units with a Requires dependency will also fail, as a dependency is missing.

So typically a fairly top-level unit (like postgresql) will declare itself that it's wanted by a top-level target (`WantedBy=multi-user.target`) which means that enabling/disabling will actually turn the service on and off, unless something else depends on it in which case disabling it does mean the explicit dependency to multi-user.target will be missing, but the unit will be pulled in wherever it's actually depended on.