Thank you for the reply. I am aware of the startup disk and I should have clarified my question better. The problem I am having is that when I work with High Sierra and switch to Snow Leopard it works fine.
When in Snow Leopard and want to switch back to High Sierra, High Sierra is not available in the startup disk (which I assume is normal because High Sierra is a later version).
When I restart from High Sierra, however, the startup is inconsistent. Sometimes it restarts with High Sierra and sometimes it restarts with Snow Leopard. Also, after an upgrade, sometimes automatically restarts with Snow Leopard. If I restart from High Sierra, I don’t think I should have to choose the startup. At least it did not happen with previous OS versions.