Non-clustered index
You can freely call these a user index. In most case scenarios, non-clustered indexes are created based on search criteria and business requirements. For example, users of your application will search based on LastName and ProductName. These attributes are not primary keys (in this universe), but you need to use the power of indexing techniques. From the SQL Server perspective, non-clustered indexes are the right choice.
Non-clustered indexes have almost the same structure as clustered indexes. The only difference is that the data rows in the tables are not sorted based on their non-clustered keys. In the non-clustered index, the data and the index are stored separately.
Non-clustered indexes are designed to improve the performance of frequently used queries that are not covered by a clustered index. If your table already has a clustered index and you need to index another column, you have no choice but to use a non-clustered index. You can achieve maximum query performance...
