It is all documented in SQL Reference Manual, chapter 2 Basic Elements of Oracle SQL, Schema Object Names and Qualifiers, Schema Object Naming Rules, Rule 12:

12. A name can be enclosed in double quotation marks. Such names can contain any combination of characters, including spaces, ignoring rules 3 through 7 in this list. This exception is allowed for portability, but Oracle recommends that you do not break rules 3 through 7.

If you give a schema object a name enclosed in double quotation marks, you must use double quotation marks whenever you refer to the object.

Enclosing a name in double quotes allows it to:

Contain spaces

Be case sensitive

Begin with a character other than an alphabetic character, such as a numeric character

Contain characters other than alphanumeric characters and _, $, and #

Be a reserved word

By enclosing names in double quotation marks, you can give the following names to different objects in the same namespace:

emp
"emp"
"Emp"
"EMP "

Note that Oracle interprets the following names the same, so they cannot be used for different objects in the same namespace:

emp
EMP
"EMP"


If you give a user or password a quoted name, the name cannot contain lowercase letters.

Database link names cannot be quoted.



SY.