How does your organization maintain an overview of all the functionalities needed within the supply chain?

Build up a repository ( task of the architects within archi (free),buzzdesign,..) with all Business processes and relations to in and external functions  .
Applications / capabilities can be related to these. 

From there you derive functional or  IT views to answer all kind of (Strategic) questions, for more technical you need from there to derive on the teams owning the Proces step/components 

This may sound oversimplified, but it is part of the strategic planning process. I have also seen companies complete end-to-end process mapping, interdependencies, swim lane or SIPOC tools, and invest in using even a third party to assess their supply chain organizational design to unearth areas of opportunity to rationalize headcount, introduce lean principles, break down silos to strengthen functional synergies, or in alignment with the operating model design continuous improvement programs and initiatives to align crucial supply chain functions better. 

It is also important to assess reporting lines. Do the functions exist all under GSC, do they roll up to the CSCO or COO? Having a clearly defined organizational chart and reporting structure is another way to view the overall functional alignment and core functions that support the company's operating model.