This is just a guess, since you are not specifying what code are you talking about, what game, who are theses "people"... so whatever answer I give is based on very little facts your question presented.
Having said that, if I had a SeneManager on my code, I would probably have Scenes as well.
A scene would be any section of my game that starts and stops at some point.
For example the main menu is a scene.
The screen that shows the logo at the beginning of my game is a scene.
The screen with highscores, and the settings menu are also scenes.
Each level of the game is a scene.
Even a pop-up dialog could be a scene.
So the scene manager is in charge of, well, managing theses things. For example in can load the logo scene, and when its over somedy tell the manager to unload Logo and load MainMenu.
The manager could have more than one scene at a time: if you load a PopUpScene you put it over the current scene. You can still see the scene below, but you are currently working in the scene at the top. When the popScene exist the manager removes it and it is left the original scene at the bottom...
The manager could also be in charge of transitioning between scenes with som sort of effect (fades and whatnot).
These are all suggestion. If you have access to some code that uses a SceneManager, dig into it and find what it does...
As for the inheritance part, SceneManager does not inherit from anything (maybe if you have a Manager class before... but... mm...).
It stands on its own.
Usually the flow is: main() starts, you create a scene manager, and you put the first scene into it so it is shown on screen...