What is the difference between Unique Key and Index with IsUnique=Yes?

Question

I have a table with a primary key, but I want two other columns to be constrained so the combination of the two is guaranteed always to be unique.

(a dumb example: In a

Answer 1

Creating a UNIQUE constraint is a clearer statement of the rule. The IsUnique attribute of the index is an implementation detail - how the rule is implemented, not what the rule is. The effect is the same though.