Описание тега unique-index
Unique-index is an index that sets one or multiple fields to be unique in a data entity
A unique index on a table guarantees that the index key contains no duplicate values and therefore ensures that every row in the table is in some way unique. Specifying a unique index makes sense only when uniqueness is a essential for the data in a table.
A unique index on a field (or set of fields) prevents two records from existing in the table with the same value in the indexed field(s). ServiceNow uses several unique indexes out-of-box to protect certain tables from duplicate record confusion.