A UNIQUE CONSTRAINT is similar to PRIMARY key, but you can have more than one UNIQUE constraint per table. When you declare a UNIQUE constraint, SQL Server creates a UNIQUE index to speed up the process of searching for duplicates. In this case the index defaults to NONCLUSTERED index, because you can have only one CLUSTERED index per table.

•  The NUMBER of UNIQUE constraints per table is limited by the number of indexes on the table i.e 249 NONCLUSTERED index and one possible CLUSTERED index.
• Contrary to PRIMARY key UNIQUE constraints can accept NULL but just once. If the constraint is defined in a combination of fields, then EVERY field can accept NULL and can have some values on them, as long as the combination values is unique.

A UNIQUE constraint is similar to PRIMARY key, but you can have more than one UNIQUE constraint per table. When you declare a UNIQUE constraint, SQL Server creates a UNIQUE index to speed up the process of searching for duplicates. In this case the index defaults to NONCLUSTERED index, because you can have only one CLUSTERED index per table.