How can I tell which monitor a display entry refers to?

Hold Option and click a display’s header in the BetterDisplay menu to briefly highlight that screen. This helps you match menu entries to physical monitors when their names look alike.

You can also open the display’s Manage Display menu and choose Identify Visually. The action is available for connected displays. For a virtual screen, Manage Virtual Screen > Identify Visually identifies its linked display while connected.

Once you have identified a monitor, you can give it a useful app label such as Left or Desk under Settings > Displays > [display] > Additional Settings… > Custom displayed name. See per-display names and shared system names for the distinction.

Visual identification helps you choose the right entry. Persistent identification is a separate matter: highlighting a screen does not change how its saved settings are matched. For that separate problem, see display identification after wake or reconnection.