When you browse your contacts in mobile app you have options to view and add Tasks and Notes related to them. Comments and Tasks you add on web will appear in the section below client details.
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To add Tasks and Notes from your phone, click on “+” icon in top right corner of your screen, the pop-up window will show at the center of your screen, allowing you to select if you want to enter new Note or Task to your dialogue. After completing the entry of the new Task or Note will show in your contact page.