Q: What does each entry in index management actually do? 
I want to have a better understanding of what each entry does and what happens when it requires a reindex.
I've read the Magento Docs fully but they aren't known for breaking down things into the most understandable pieces. I know the difference/functionality of update on save and scheduled. Just not quite sure what it is doing behind the scenes.   
Specifically, I do not understand what the entries for :
Rebuild Design Config index and Rebuild customer grid index are for.
Is there a directory I can locate these files in and change them manually?
Any help will be greatly appreciated. 

A: Magento use flat table customer_grid_flat for customer grid so when you rebuild index for customer grid it will take data from eav tables and put data here in flat table for customer.
I can't see design config in my setup. It seems  to be available after some changes.
