You will find the link who explain the process :
Little part of the post:
Included columns is a feature of SQL Server (since version 2005) related to indices.Thanks to it you can attach additional fields to an index that are not used for searching (so they don’t have to be maintained so expensively) but that can be used by database server to return data. If all columns being returned by a query are included in an index, database server can directly return data and it doesn’t have to touch the table itself (and that saves time, of course).
Included columns also have additional advantages:
- can be added to a unique index without influencing uniqueness
- support even (some) types that can’t be used in normal indices
- don’t count to the limit of number of index columns (16)
- don’t count to the limit of index size (900 bytes)
The following (logical) restriction apply:
- are always at the end of index
- the index must contain at least one “normal” (key) column
- can’t be used in clustered indicesOf course, even included columns have impact to index size so they can slow down operations with the index and consume disk space.