I'm not sure I follow your question exactly... But:
In the mechanical world we have assembly drawings and subassembly drawings. The same design principles apply to both. The only difference is that a subassembly can conveniently be assembled, stored, handled, and installed as a single component of the larger assembly. For example, Your car's gas tank is an assembly of several components. It is also a component of the entire car assembly. The fuel pump inside your gas tank is itself another subassembly. Each of these components and subassemblies would have their own drawing and can be designed on its own.
Is that what you were asking?