Instant Payment Notifications (IPN) allow your applications to receive notifications from PayPal on payments made. This means that your application can fulfill an order automatically upon receiving such a notification. However, when you get your second application up with its own IPN you suddenly find out that PayPal lets you set only one Notification URL.
If you are moving from one membership plugin to MemberMouse, you can simply reuse the IPN you were using in your previous membership plugin.
For some clients, there is a need to use an IPN on multiple sites. Paypal does not have a feature that allows you to set up a site specific IPN. So if this situation comes up, here is an article that explains how you can set up Multiple IPN receivers in PayPal. Remember, if you are just switching from one membership plugin to MemberMouse, you do not need to utilize this method. You can just reuse your PayPal account IPN within MemberMouse