This method returns the resource key for a parsed resource expression. The resource key is the value specified after the comma (,) in an expression of the form <%$ Resources: ClassKey, ResourceKey %>. The ResourceExpressionFields.ResourceKey property identifies a particular key/value pair within the resource file mapped to the class key. For example, if a file named Financial.resx contains a key called Currency, that value could be specified with the resource expression <%$ Resources: Financial, Currency %>. After parsing, the ResourceExpressionFields.ResourceKey property would return "Currency" as its value.