There are three ways to add entries to the logbook.
1. Click on Add Note on the main screen (which uses the current time and glucose values)
2. Perform an NFC scan which also uses the current time and glucose values.
3. Go into the Logbook for a particular day and click Add Note and it will ask for the time, but does not record a glucose value in the logBook entry.
Option 1 is the one I use most often, especially for recording carbs and insulin dosages.
Option 3 is when I forget to enter meal details at the time and have to do it later.
I very rarely do an NFC scan.