Each database object has a name. You can use a quoted or nonquoted identifier to indicate the name of an object in an SQL statement.
In fact, you can use a quoted or nonquoted identifier to name any database object.
A quoted identifier begins and ends with double quotation marks (" "). If you use a quoted identifier to name a schema object, you must add the double quotation marks when you reference the object.
An nonquoted identifier carries no punctuation mark.
Database names, global database names, and database link names are case-insensitive and stored in uppercase. If you specify a quoted identifier for a name of this type, the quotation marks are silently ignored. For more information about how to name a user, see CREATE USER.