Inside Macintosh: AppleScript Language Guide / Part 2 - AppleScript Language Reference

Chapter 3 - Values / Value Class Definitions

Constant

A value of class Constant is a reserved word defined by AppleScript or an application in its dictionary. Applications define sets of values that can be used for parameters of a particular command; for example, the value of the saving parameter of a Close command must be one of the three constants yes, no,
and ask.

LITERAL EXPRESSIONS

yes
no
ask

PROPERTY

Class

The class identifier for the object. This property is read-only,
and its value is always constant.

ELEMENTS

None

OPERATORS

The operators that take values of class Constant as operands are &, =, ≠, and As.

COERCIONS HANDLED

AppleScript supports coercion of a constant to a single-item list.

NOTES

Constants are not strings, and they must not be surrounded by
quotation marks.

You cannot define your own constants; constants can be defined only
by applications and AppleScript.