What is "single-restriction"

While the UNIQUE Constraint and PRIMARY KEY Constraint impose exclusivity, use the UNIQUE Constraint instead of the PRIMARY KEY Constraint when imposing the exclusivity of a column, or a combination of columns, which is not a primary key.

Several UNIQUE Constraint can be defined in a table, whereas only one PRIMARY KEY Constraint can be defined in a table. In addition, unlike the PRIMARY KEY Constraint, the UNIQUE Constraint allows a NULL value. However, as with any value that participates in a Constraint UNIQUE, only one null value is allowed per column. A UNIQUE Constraint can be referenced by a FOREIGN KEY Constraint.