UNIQUE argument for INDEX creation - what's for?

Question

Why does INDEX creation statement have UNIQUE argument?

As I understand, the non-clustered index contains a bookmark, a pointer to a row, which should be unique to dist

Answer 1

The UNIQUE clause specifies that the values in the column(s) must be unique across the table, essentially adding a unique constraint. A clustered index on a table specifies that the ordering of the rows in the table will be the same as the index. A non-clustered index does not change the physical ordering, which is why it is OK to have multiple non-clustered but only one clustered index. You can have unique or non-unique clustered and non-clustered indexes on a table.