Identifiers
Identifiers can refer to the values of message headers and properties, but not to the message body. Identifiers are case-sensitive.
Basic Syntax
An identifier is a sequence of letters and digits, of any length, that begins with a letter. As in Java, the set of letters includes _ (underscore) and $ (dollar).
Illegal
Certain names are exceptions, which cannot be used as identifiers. In particular, NULL, TRUE, FALSE, NOT, AND, OR, BETWEEN, LIKE, IN, IS, and ESCAPE are defined to have special meaning in message selector syntax.
Value
Identifiers refer either to message header names or property names. The type of an identifier in a message selector corresponds to the type of the header or property value. If an identifier refers to a header or property that does not exist in a message, its value is NULL.