is defined as MM::Engine.
is defined as MM::Services::Service, the Service's base class, which was previously known as ServiceSystem and SubSystem.
is defined as a ECS-style EnTT::registry. It contains Entities, Components and Systems.
is defined as a ECS Entity, see EnTT.
is defined as a ECS-style System. In this case, it's a free function.
is defined as a State of enabled and disabled Services. (see ScreenDirector)