Currently, the relations are a mess. "Optional Dependency" is often used for compatible mods instead of mods that add new features to the original mod itself. "Included In" is used more for modpacks and not packaged mods (buildcraft-all comes to mind for Minecraft).
The way I envision it, we would have three tabs, each with their own headings (as noted by the lettering):
Libraries and Frameworks
Recommended In (for client mods)