When experimenting with Joplin, for example when developing a plugin or trying a theme, you might want to run Joplin in development mode. Doing this will open a second copy of Joplin using a different profile with test notes and notebooks, so you can experiment without risking changing or deleting your data.
To enable Development Mode, go to Help > Copy dev mode command to clipboard. This will copy a command to your clipboard. Paste this command in a shell / terminal to run a development version of the app.