When I install SQL Server 2008 Express in prompts me to create an instance and aborts if I don't. Then I see that information in an entry in Sql Server Configuration Manager on SQL Server Services. What is a SQL Server instance?
An SQL Server instance is a complete SQL server and you can install many instances on a machine but you can have only 1 default instance.
An SQL Server instance has its own copy of the server files, databases and security credentials.
This url may help you
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