Key name. This follows the same syntax rules as other expression entity names: It must begin with an alpha character (A-Z or a-z) or an underscore (_). The rest of the characters must be alpha, numeric (0-9) or underscores. It cannot be re or xp (reserved for regular and XPath expressions). It cannot be an expression reserved word (e.g. SYS or HTTP). It cannot be used for an existing expression object (HTTP callout, patset, dataset, stringmap, or named expression).