Toolbar Action Buttons are simply customizable shortcuts to various functions of GPX Viewer. You can choose various functions which will be available on top toolbar of map screen of GPX Viewer. Image below is highlighting Toolbar Action Buttons in GPX Viewer.

You can customize Toolbar Action Buttons by tapping on … toolbar action button in top toolbar of map screen, then scrolling down in menu and tapping on Customize menu item. If you have already removed this … toolbar action button, just open left main menu, then tap on Settings, then go to User Interface and tap on Toolbar Action Buttons. In this screen you can customize Toolbar Action Buttons as is shown in video below.