Allows for the use of dynamic variables in the condition
Many conditions support dynamic variable expansion in their attributes or content. Where supported, an embedded reference of the form $<variable-name>$ is replaced with the value of the local or global variable with the given name. $<variable-name>$ must be legal variable name. For information on what is legal XML name, see W3C Extensible Markup Language (XML).
If the given variable does not exist, the reference is replaced with the empty string. Where it is desirable to use a single $ and not have it interpreted as a variable reference, it should be escaped with an additional $ (for example, You owe me $$100.00).