Select NAP ALARM

Hallo all! I have set up my sleep time same for all days, from 11 pm to 6 am. 

In the afternoon I take a nap, and my question is what does "Select Nap Alarm to set nap alarms to play as a tone, vibration, or both." means. 

I do not set any nap duration, it can be from 30' to 1 hour and a half. But I detect no tone or vibration.

Thanks