ER-Designer Indexes (Keys in the logical model). The screen shot on the right shows the SQL generated on a specific table.
There are two primary uses for Indexes. One is to speed up read (query) access to a table when a specific set of columns are used in the query. Like a FK and a name.
The other is use is to enforce a uniqueness on one or more columns. That is a combination of columns. Using Indexes to enforce uniqueness is very similar to using a unique constraint.
The screen shots below show the User Interface in the Entity Editor for Indexes. The user can select any combination of columns to add to the index. Indexes can be tagged as unique.