Here is a list of sync icons you will encounter in Nebo, and what they mean:
At the top of the Notebooks section
Tap to connect your cloud account.
No internet connection or Sync disabled in the settings.
Next to your notebook
Tap to sync this notebook.
Tap to download a notebook from the cloud.
This notebook will be deleted at next sync: To prevent this action from happening, open the notebook and add some content in it, like a word, an image, anything. If it was a page you deleted, add content in this one page. When this is done, your notebook is considered restored, but you need to tap Sync on this device and then on the device you want to have it back, to also restore it over there.
Your sync wasn't successful: Sometimes Nebo detects a conflict between the notes stored on your device and the cloud. When it occurs, you are asked via a pop-up window to choose between the local and the cloud version.
Please be aware that once you have selected your option, you cannot go back and change it – only the Dropbox sync feature lets you do so via thanks to its history mechanism