" Notification Settings" stands for sending mails to customers for credit updates. The customer will get notification by mail for getting credit or for spending credit 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 examples: Mailgun, Sparkpost, Mandrill
We can help you to integrate Gmail, Klaviyo, Office 365 as well.