[HELP] DRO Questions

[Cross Slide]

I just finished installing a DRO on my lathe and my mill.
I have not had a chance to do any machining using the DROs yet. 
Since I'm new to machining I still learning the terms and procedures. 

The DRO I installed is a BLU-DRO which is a Bluetooth setup using the Touch DRO app on a android tablet. The scales are Igaging scales. 

The first question I have is the Absolute / Incremental button. 
What does each mean and when to I want to use one over the other?

   

Thanks
Trevor