The meaning of engine is getting a little blurred these days. Often when people talk of engines they think of entire game creation tools such as Unreal or Unity. While these are engines, the term didn't always require a tool. Game engines such as Id Software's Quake Engine or Valve Corporation's Source engine existed independently of tools, although the latter did have tools including the Hammer Editor for creating levels.

The term engine is also used to refer to components within the larger code base. This includes things like a rendering engine, audio engine, or physics engine. Even these can be created completely separate from a larger code base. Orge 3D is an open source 3D graphics engine, while the Havok Physics engine is proprietary software created by the Havok company and used in many games.

So, when we talk about the engines or systems of the Mach5 engine, we are simply referring to groups of related code for performing a specific task.