When you handle a ticket the requester will be notified by mail on important occurrences, e g ticket has a new comment, ticket status is changed, a solution is found, ticket is closed, etc. This is handled automatically in the system and you as an Agent don´t have to take any action to send notifications. Which notifications that should be sent is controlled by system settings, and could vary in different systems.
An incoming mail to NSP that concerns an already existing ticket (e g a notification reply), has a function to find this ticket and redirect the mail correct. The mail will be added as a comment to the ticket.
NSP system automatically adds an identifier to the subject of all outgoing mails that are sent from a ticket, composed by a prefix called Ticket Identifier Prefix, eg “REFID” and the ticket id. The identifier could look like “REFID: REQ0000456”. If the requester replies to such a mail it will contain the correct identifier to find the correct ticket without any additional manual input.