Because of limitation to 5 icons, last icon in the mobile application is selectable. When you see an icon with dots, it means that you can access so called More menu. Seen on the picture in the bottom right corner.
More menu is where all selectable options reside (Sensors, Maintenance, Docs,..). Options depend on the type of the account and access type. More menu is default option on newly created accounts. When you tap on it new window will open with options.
When for example Sensors are selected, the 5th icon changes accordingly.
Important! When no icon for More menu is visible you can access it by double tapping the 5th icon. So in order to change back to Docs, double tap the Sensors icon - above case.