Need Help in understanding the outlook addin for office365

I am trying to convert VSTO outlook plugin to office 365 outlook addin. I need help understanding the difference between userproperties (VSTO) and customproperties offered by addin. I want to be sure if these two stores data at the same place. this is because once the data is saved. A backend service needs to read the same and perform some operations. please bid if you have knowledge about the task Thank you