Issue is an incoming query from your Customer, usually via email or from the Contact section of your website.
If you are selling Items under warranty or with an Annual Maintenance Contract (AMC), you can manage. warranty claims of customers in ERPNext
A Maintenance Visit is a record of a visit made by an engineer to a Customer’s premise usually against a Customer Issue.
This is a tool that is used to plan a calendar of activities for maintenance of various machines or equipments.
A service-level agreement is a commitment between a service provider and a client.