The behavior of modular multilevel converters (MMCs) is, to a large degree, determined by their control and protection (C&P) software. Understanding and managing the functionalities of the C&P software are becoming relevant in high-voltage direct-current (HVDC) systems with setups ranging from electrical proximity of single-vendor systems, connections on the same AC or DC busbars to more advanced multi-vendor setups with split C&P software as done in China. Some concept-level ideas for clustering C&P software functions have been put forward. However, the organizational and management aspects have not been discussed in the available literature. This paper presents the model-based systems engineering (MBSE) methodology as a possible approach to organizing software functions and their implications and interactions in multi-vendor HVDC setups. Some practical aspects are illustrated using an example, and future developments and synergies with other tools are discussed. The presented ideas are meant to stimulate discussions in the field of multi-vendor HVDC interoperability - a domain that will have to manage complex software and hardware, such as to deliver the anticipated unprecedently high number of HVDC systems.