Ans: " Notification settings" stand for send mail to customers for credit update. Customer will get notification by mail for getting credit or spent credit time from his/her account.
Credit for register/signup: you can set custom notification mail for your customers who will register/signup to your store.
(Note: Before that, you have to enable "store credit" option through "app settings" option, and then set value for register customer through "credit setting")
Hosted Mail platform ex: MAilgun, Sparkpost, mandrill
We can help you to integrate gmail, klaviyo, office 365 as well.